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REFRACTION 



HOW TO REFRACT 



THORINGTON 



BY THE SAME AUTHOR. 

Retinoscopy (The Shadow Test) in the Determination 
of Refraction at One Meter Distance with the Plane 
Mirror. 54 Illustrations, a number of which are in 
Colors. Fifth Edition. Cloth, net, $1.00 

From The Medical Record, New York. 

" It presents a clear, terse, and thorough exposition of an 
objective method of determining refraction errors which is de- 
servedly increasing in popularity. In our opinion the author is 
amply justified in declaring that its great value in nystagmus, young 
children, amblyopia, aphakia, and in examining illiterates and the 
feeble minded, cannot be overestimated, and we agree with him in 
reminding those who attempt retinoscopy, fail, and ridicule it, that 
the fault is behind and not in front of the mirror. The book is well 
printed and usefully illustrated." 

The Ophthalmoscope and How To Use It. With De- 
scriptions and Treatment of the Principal Diseases of 
the Fundus. With 12 Colored Plates and 73 other 
Illustrations. Cloth, net, $2.5.0 



REFRACTION 



AND 



HOW TO REFRACT 

INCLUDING SECTIONS ON OPTICS, RETINOSCOPY, THE 
FITTING OF SPECTACLES AND EYE-GLASSES, ETC. 



JAMES THORINGTON, A.M., M.D., 

rROFHSSOR OF DISEASES OF THE EYE IN THE PHILADELPHIA POLYCLINIC AND COLLEGB 

FOR GRADUATES IN MEDICINE; MEMBER OF THE AMERICAN OPHTHAL- 

MOLOGICAL SOCIETY; FELLOW OF THE COLLEGE OF 

PHYSICIANS OF PHILADELPHIA, ETC. 



jfourtb Bfcition 



TWO HUNDRED AND TWENTY ILLUSTRATIONS 

THIRTEEN OF WHICH ARE COLORED 



PHILADELPHIA 
P. BLAKISTON'S SON & CO 

I O I 2 WALNUT STREET 
1909 



<\ 



';> 




LIBRARY of CONGRESS 


Two Copies Received 


feb 24 iaoy 


_ Oopyrignt entry 
CLASS Cc, XXc, No, 



Copyright, 1909, by P. Blakiston's Son & Co. 



First Edition, 1899. Second Edition, 1900; Reprinted January 1902. 

Third Edition, 1904; Reprinted February and November, 1906, 

September, 1907, May, 1908. Fourth Edition, 1909. 



WM. F. FELL COMPANY 

Electrotypers and Printers 

1220-24 Sansom Street 

Philadelphia, Pa. 




Horizontal Section of the Right Eye. — (Landots.) 
Cornea, b. Conjunctiva, c. Sclerotic, d. Anterior chamber containing the 
aqueous humor, e. Iris. ff. Pupil, g. Posterior chamber. /. Petit's canal. 
j. Ciliary muscle, k. Corneoscleral limit, i. Canal of Schlemm. m. Choroid. 
n. Retina, o. Vitreous humor. No. Optic nerve, q. Nerve-sheaths, p. Nerve- 
fibers. Ic. Lamina cribrosa. k. Crystalline lens. or. Ora serrata. pc. Ciliary 
processes. The line, O A, indicates the optic axis ; 6* r, the axis of vision ; r, the 
position of the fovea centralis. Kn. Nodal point, x. Equator of lens. t. Ex- 
ternal rectus muscle, s. Internal rectus muscle. Z. Optic nerve-sheath. H. 
Sclerotic. 




Muscles of the Eye. Tendon or Ligament of Zinn. 
Tendon of Zinn. 2. External rectus divided. 3. Internal rectus. 4. Inferior 
rectus. 5. Superior rectus. 6. Superior oblique. 7. Pulley for superior oblique. 
8. Inferior oblique. 9. Levator palpebrae superioris. 10, 10. Its anterior expan- 
sion. 11. Optic nerve. 



PREFACE TO FOURTH EDITION. 



The fourth edition of this text-book has been carefully 
revised and an appendix has been incorporated which con- 
tains additional information on the subject of Refraction, as 
also descriptions and illustrations of new instruments. 

The writer wishes to express his sincere appreciation of 
the generous reception which has been extended to this book 
and trusts that this edition may prove as meritorious as its 
predecessors. 

J. T. 

Philadelphia, February, 1909. 



PREFACE TO FIRST EDITION. 



This book has been written at the request of the many 
students who have attended the author's lectures on 
"Refraction" at the Philadelphia Polyclinic; and while it 
is intended for all beginners in the study of Ophthalmology, 
yet it is especially for those practitioners and students who 
may have a limited knowledge of mathematics and who can 
not readily appreciate the classic treatise of Donders. 

In the preparation of the manuscript and in arranging these 
pages the writer has planned to be systematic and practi- 
cal, so that the student, starting with the consideration of 
rays of light, is gradually brought to a full understanding 
of optics ; and following this, he is taught what is the stan- 
dard eye, and then is given a description of ametropic eyes, 
with a differential diagnosis of each, until finally he is told 
how to place lenses in front of ametropic eyes to make 
them equal to the standard condition. 

By being dogmatic rather than ambiguous, with occa- 
sional repetitions to avoid frequent references, and by simple 
explanations and a definite statement of facts, the writer has 
aimed to make the text more concise and comprehensive 
than if encumbered with lengthy mathematic formulas or 
with any discussion of disputed points. 

The chapter on Retinoscopy embraces descriptions of that 
method of refracting, both with the plane and with the 
concave mirror ; but no matter how carefully expressed, the 



Xll PREFACE. 

student will frequently confuse the two, and he is therefore 
referred to the author's manual on " Retinoscopy with the 
Plane Mirror." 

Of the two hundred illustrations used to elucidate this 
work, nearly all are newly made, and were drawn or photo- 
graphed by the author. Those in colors, on page 145, and 
the diagrams of astigmatic eyes, as also several others, are 
original. 

The author desires to tender his thanks to Dr. Helen 
Murphy, of Philadelphia, and to Dr. J. Ellis Jennings, of 
St. Louis, Mo., for many valuable suggestions. 

120 S. 1 8th St., Philadelphia, Pa. 
November, i8gg. 



CONTENTS 



CHAPTER I. 

PAGE 

Optics, 9 

CHAPTER II. 

The Eye. — The Standard Eye. — The Cardinal Points. — Vis- 
ual Angle. — Minimum Visual Angle. — Standard Acute- 
ness of Vision. — Size of Retinal Image. — Accommodation. 
— Mechanism of Accommodation. — Far and Near Points. 
— Determination of Distant Vision and Near Point. — 
Amplitude of Accommodation. — Convergence. — Angle 
Gamma. — Angle Alpha, 59 

CHAPTER III. 
Ophthalmoscope. — Direct and Indirect Methods, 88 

CHAPTER IV. 
Emmetropia. — Hyperopia. — Myopia, lo ^ 

CHAPTER V. 
Astigmatism, or Curvature Ametropia. — Tests for Astigma- 
tism, 122 

CHAPTER VI. 
Retinoscopy, j-6 

CHAPTER VII. 
Muscles, ,_*- 

CHAPTER VIII. 
Cycloplegics. — Cycloplegia. — Asthenopia. — Examination of 

the Eyes, 20 g 

xiii 



XIV CONTENTS. 

CHAPTER IX. 

PAGE 

How to Refract, 228 

CHAPTER X. 
Applied Refraction, 244 

CHAPTER XI. 
Presbyopia. — Aphakia. — Anisometropia. — Spectacles, .... 271 

CHAPTER XII. 
Lenses, Spectacles, and Eye-glass Frames. — How to Take 
Measurements for Them and How They Should be 
Fitted, 297 

APPENDIX, 307 

INDEX, . . . 317 



LIST OF ILLUSTRATIONS. 



FIG. PAGE 

1. Illustrating Intensity of Light, io 

2. Convergent Pencil, ... 1 1 

3. Convergent Pencil, 1 1 

4. Divergent Pencil, 12 

5. Reflection, 13 

6. Reflection from Plane Mirror, 13 

7. Lateral Inversion, 14 

8. Reflection from Concave Mirror, . . 15 

9. Erect Image Formed by Concave Mirror, 16 

10. Inverted Image Formed by Concave Mirror, 17 

11. Image Formed by Convex Minor, 18 

12. Perpendicular to Plane Surfaces, 19 

13. Refraction, 19 

14. Critical Angle, 20 

15 and 16. Angle of Refraction, 21 

17. Density, 21 

18. Index of Refraction, 22 

19. Maximum Deviation, 23 

20. Minimum Deviation, 23 

21. Angle of Deviation, 24 

22. Displacement, 25 

23. Centrad, 25 

24. Prism Diopter, 25 

25. Neutralization of Prisms, 26 

26. Correction of Diplopia, 29 

27. 2S, and 29. Convex Lenses, 30 

30, 31, and 32. Concave Lenses, 31 

^. Prism Formation of a Convex Lens, 31 

34. Prism Formation of a Concave Lens, . 31 

35. Parallel Rays Passing Through a Convex Lens, 32 

36. Parallel Rays Passing Through a Concave Lens, 33 

37. Conjugate Foci, 34 

38. Ordinary Foci, 35 

39. Negative Focus, 36 

40. Nodal Points, 36 

41. Optic Center, 37 

42. Inverted Image Formed by a Convex Lens, ^S 

43. Erect Magnified Image Formed by a Convex Lens, 40 

44. Image Formed by a Concave Lens, 40 

45 and 46. Cylindric Lenses, 44 

47. Cylinder Axis, 44 

48. Parallel Rays Passing Through a Convex Cylinder, ...... 44 

xv 



XVI LIST OF ILLUSTRATIONS. 

FIG. PAGB 

49. Parallel Rays Passing Through a Concave Cylinder, 45 

50. Trial Case, 46 

51 and 52. Trial- frames, 47 and 48 

53. Combining Sphere and Cylinder, 50 

54, 55, and 56. Finding Optic Center of a Lens, . 55 

57 and 58. Finding Cylinder Axis, 56 

58, 59, and 60. Action of a Cylinder, .... ...... 56 and 57 

61. Standard Eye, bo 

62. Angle of View, 61 

63 and 64. Size of Retinal Image, 62 

65. Minimum Visual Angle, 63 

66 and 67. Five-minute Angle, 64 

68. Retinal Image in the Standard and Ametropic Eyes, 65 

69. Crystalline Lens at Rest and Accommodating, 68 

70. Accommodation, 69 

71. Hyperopic Eye at Rest, 71 

72. Myopic Eye at Rest, 72 

73. Randall's Test-letters, 74 

74. Wallace Test-letters, 75 

75. Illiterate Card, 75 

76. Kindergarten Card, 76 

77. Gould's Test-letters, 77 

78. Gothic Type for Testing the Near Point, ^ . . . . 81 

79. Block Letters for Testing the Near Point, 82 

80. Meter Angle of Convergence, 84 

81. Angle Gamma, 85 

82. Positive Angle Gamma, 86 

S^. Negative Angle Gamma, 87 

84. Loring Ophthalmoscope, 89 

85. Direct Ophthalmoscopy, k 90 

86. Emmetropia with the Ophthalmoscope, 95 

87. Hyperopia with the Ophthalmoscope, 96 

88. Myopia with the Ophthalmoscope, 97 

89. Indirect Ophthalmoscopy, 99 

90. Condensing Lens, 99 

91 and 92. Luminous Ophthalmoscope, 102 

93. Emmetropia, 103 

94. Emmetropic and Ametropic Eyes 23 mm. Long, 104 

95. Hyperopic Eye at Rest, 107 

96. Hyperopic Eye Refracted, 107 

97. Parallel Rays Entering a Myopic Eye, 113 

98. Myopic Eye at Rest, 114 

99. Myopic Eye Refracted, 114 

100. Astigmatic Lens, 123 

101. Simple Hyperopic Astigmatism, 126 

102. Simple Myopic Astigmatism, 127 

103. Compound Hyperopic Astigmatism, 127 

104. Compound Myopic Astigmatism, 128 

105 and 106. Mixed Astigmatism, 129 

107. Symmetric Astigmatism 13° 

108. Asymmetric Astigmatism, 130 

109. Astigmatism with the Rule, 131 



LIST OF ILLUSTRATIONS. XV11 

FIG. PAGE 

no. Astigmatism against the Rule, 131 

in. Placido's Disc, 135 

112. Stenopeic Slit, 135 

113. Green's Astigmatic Charts, 137 

114. Astigmatic Clock-dial, 138 

115. Astigmatic Clock-dial in Black, 139 

116. Author's Pointed Line Test, 141 

117. Perforated Disc, 142 

118. Pray's Letters, 142 

119. Scheiuer's Disc, 143 

120. Scheiner's Disc in Hyperopia, 143 

121. Scheiner's Disc in Myopia, ' 144 

122 and 123. Cobalt-blue Glass, 145 

124. Refrangibility of Cobalt-blue Glass, 146 

125 to 136, inclusive. The Diagnosis of the Different Forms of Ame- 
tropia with Cobalt-blue Glass, 147 

137. Thomson's Ametrometer 149 

138 and 139. Ophthalmometer, 150 and 151 

140 and 141. Mires or Targets, 152 

142. Indirect Ophthalmoscopy, 155 

143. Author's Schematic Eye, 156 

144. Point of Reversal, 157 

145 and I46. Author's Mirror with Folding Handle, 158 

147. Author's Iris Diaphragm Chimney, 159 

148. Position of Light and Mirror, 160 

149. High Myopia as Seen with the Concave Mirror, 1 61 

150. Hyperopia as Seen with the Concave Mirror, ... . . . . 161 

151 and 152. Rate of Movement of Retinal Illumination in Hyperopia 

and Myopia, 164 and 165 

153. Retinal Illumination in Emmetropia, 166 

154. Band of Light, 169 

155. Axonometer, 170 

156. Scissor Movement, 172 

157. Positive Aberration, 173 

158. Negative Aberration, 173 

159 and 160. Reisner Retinoscope, 174 

161 and 162. Luminous Retinoscope, 175 

163. Homonymous Diplopia, 178 

164. Heteronymous Diplopia, 179 

165. Scale for Testing Lateral Insufficiency, 187 

166 and 167. Maddox Rods, 188 

168. Rotary Prism of Risley, 189 

169. Phorometer, 190 

170. Strabismometer, .... 200 

171. Angle of Deviation in Strabismus, 201 

172. Monocular Blinder, 203 

173. Worth's Amblyoscope, 204 

174. Aphakia, 278 

175 and 176. Franklin Bifocals, 284 

177. Mork's Bifocals, 284 

178 to 184, inclusive. Cement Bifocals, 285 and 286 

185 and 186. Acromatic Bifocals, 286 



XV111 LIST OF ILLUSTRATIONS. 

FIG. PAGE 

187 and 188. Solid Bifocals, 287 

189 to 193, inclusive. Half Lenses 288 

194. Toric Lenses, 290 

195 and 196. Trifocals, 295 

197 to 206, inclusive. Different Sizes and Shaped Lenses, . 298 and 299 

207. Measuring Interpupillary Distance, 301 

208 and 209. Fitting of Spectacle Bridge, 302 

210. Measurement of Bridge, 303 

211. Measurement for Spectacles, 304 

212. Measurement for Eye-glasses, 304 

213. Distance Frames, . . 305 

214. Near Frames, 305 

215. Measurements for Guards, 305 

2i6. Metric Test Letters, 309 

217. Metric Test Letters, 309 

218. Metric Lines, 312 

219. Axonometer, 3 X 5 

220. Luminous Retinoscope, 316 



REFRACTION 



HOW TO REFRACT. 



CHAPTER I. 

OPTICS. 

Optics (from the Greek 0-rojj.at, meaning "to see") is 
that branch of physical science which treats of the nature 
and properties of light. 

Catoptrics (from the Greek zdroircpov, meaning " a mir- 
ror") and dioptrics (from the Greek dioTtrpov, meaning 
14 to see through ") are subdivisions of optics ; the former 
treating of incident and reflected rays, and the latter of the 
refraction of light passing through different media, such as 
air, water, glass, etc., but especially through lenses. 

Light. — Light may be defined as that form of energy 
which, acting upon the organs of sight, renders visible the 
objects from which it proceeds. This form of energy is 
propagated in waves in all directions from a luminous body, 
and with a velocity in a vacuum of about 186,000 miles a 
second. In the study of a luminous body, such as a candle-, 
lamp-, or gas-flame, the substance itself must not be con- 
sidered as a single source of radiation, but as a collection 
of minute points, from every one of which waves proceed 
in all directions and cross one another as they diverge from 
their respective points. The intensity of light decreases 

9 



10 REFRACTION AND HOW TO REFRACT. 

as the square of the distance from the light increases : for 
example, if an object is twice as far from a luminous body 
as another of the same size, it will receive one-fourth as 
much light as the latter. Figure I shows two cards, one 
is twice as far from the light as the other and receives only 
one-fourth as much light as the card nearest to the light. 

Ray. — Ray (from " radius ") is used in optics in prefer- 
ence to wave, and means the smallest subdivision of light 
traveling in a straight line. Rays of light are considered 
as incident, emergent, reflected, refracted, divergent, par- 
allel, and convergent. 



Fig. I. — Illustrating Intensity of Light. 

Incident Rays. — Rays of light are said to be incident 
when they strike the surface of an object. (See Fig. 8.) 

Emergent Rays. — Rays of light are emergent when they 
have passed through a transparent substance. (See Fig. 13.) 

Reflected Rays. — Rays of light are reflected when they 
rebound from a polished surface. (See Fig. 8.) 

Refracted Rays. — A ray of light undergoes refraction 
when it is deviated from its course in passing through any 
transparent substance. 

Divergent Rays. — Rays of light proceed divergently 
from any luminous substance, but, in the study of refrac- 
tion, only those which proceed from a point closer than six 
meters are spoken of as divergent. (Fig. I.) 

Parallel Rays. — The greater the distance of any lumin- 
ous point, the more nearly do its rays approach to paral- 



OPTICS. 



II 



lelism ; this is evident in a study of rays coming from such 
distant sources as the sun, moon, and stars. For all prac- 
tical purposes in the 
study of refraction, 
rays of light which 
proceed from a dis- 
tance of six meters or 
more are spoken of 
as parallel, although 
this is not an absolute 

fact, as rays of light 

at this distance still 

maintain a slight 

amount of divergence. 

If the pupil of the 

emmetropic eye is represented by a circular opening four 

millimeters in diameter, then rays of light from a luminous 

point at six meters (6000 mm.) will have a divergence of 

^JL_ when they enter such a pupil. 

Convergent Rays. — Convergent rays are the result of re- 




Fig. 2. — Parallel Rays Reflected by a Concave 
Mirror Forming a Convergent Pencil. 




Fig 1 —Parallel Rays Refracted by a Convex Lens Forming a Convergent 

Pencil. 

flection from a concave mirror or refraction through a con- 
vex lens. (See Figs. 2 and 3.) 

A Beam. — This is a collection or series of parallel rays. 
(See Fig. 3.) 

A Pencil- — A pencil of light is a collection of convergent 



12 



REFRACTION AND HOW TO REFRACT. 



or divergent rays. Convergent rays are those which tend 
to a common point (see Fig. 3), whereas divergent rays are 
those which proceed from a point and continually separate 
as they proceed. (See Fig. 4.) This point is called the 
radiant point. 

A Focus. — This is the point of a convergent or divergent 
pencil ; the center of a circle ; the point to which converg- 
ing rays are directed. 

A Positive or Real Focus. — This is the point to which 

rays are directed after passing through a convex lens or 

after reflection from a concave mirror. (See Figs. 3 and 2.) 

A Negative or Virtual Focus. — This is the point from 

which rays appear to 

diverge after passing 

through a concave lens 

(see Fig. 36), or after 

reflection from a convex 

mirror, or after refraction 

through a convex lens 

when the light or object 

is closer to the lens than 

its principal focus (see 

Fig. 43), or after reflection from a concave mirror when 

the light or object is closer to the mirror than its principal 

focus. (See Fig. 9.) 

The principal phenomena of light are absorption, reflec- 
tion, and refraction. 

Absorption. — Rays of light from the sun falling upon 
the green grass are partly absorbed and partly reflected. 
The grass absorbs some of the rays and sends back or 
reflects only those rays which together produce the effect 
of green. A piece of red glass owes its color to the fact 
that it transmits only that portion of the light's rays whose 
combined effect upon the retina is that of red. The relative 




Fig. 4. — Illustrating a Divergent Pencil. 



OPTICS. 



13 



proportion of absorption and reflection of rays of light 
depends greatly upon the quality of the surface — whether 
light colored or polished, or dark colored or rough. 

Reflection. — From the Latin reflectere, " to rebound." 
This is the sending back of rays of light by the surface on 
which they fall into the medium through which they came. 
While most of the rays falling upon the surface of a trans- 
parent substance pass through it, with or without change 
in their direction, yet some of the rays are reflected, and it 
is by these reflected rays that surfaces are made visible. 




D 
Fig. 5. 




Fig. 6. 



A substance that could transmit or absorb all the rays of 
light coming to it (if such a substance existed) would be 
invisible. Reflection, therefore, always accompanies refrac- 
tion, and, if one of these disappear, the other will disappear 
also. 

Laws of Reflection. — (1) The angle of reflection is 
equal to the angle of incidence. (2) The reflected and in- 
cident rays are in the same plane with the perpendicular to 
the surface. (See Fig. 5.) 

If A B represent a polished surface and I the incident 
ray, then P D I is the angle of incidence ; R being the re- 



14 



REFRACTION AND HOW TO REFRACT. 



fleeted ray, then P D R, equal to it, is the angle of reflection. 
I D, P D, and R D lie in the same plane. 

A reflecting surface is usually a polished surface (a 
mirror), and may be plane, concave, or convex. 

Reflection from a Plane Mirror. — Rays of light are 
reflected from a plane mirror in the same direction in which 
they fall upon it : if parallel, convergent, or divergent be- 
fore reflection, then they are parallel, convergent, or diver- 
gent after reflection. An object placed in front of a plane 
mirror appears just as far back in the mirror as the object 

is in front of it. (See 
Fig. 6.) 

A B represents a plane 
mirror with E F, rays 
from the extremes of the 
object I, reflected from 
the mirror A B, and meet- 
ing at the observer's eye 
as if they came from the 
object I in the mirror. 
(See Visual Angle, p. 
62.) The apparent dis- 
tance of the object I from the observer is equal to the 
combined length of the incident and reflected rays. 

The appearance of an image in a plane mirror is not 
exactly the same as that of the object facing the mirror ; 
it undergoes what is known as lateral inversion. This is 
best understood by holding a printed page in front of a 
plane mirror, when the words or letters will read from right 
to left. (See Fig. 7.) An observer facing a plane mirror 
and raising his right hand, his image apparently raises the 
left hand. 

Tilting a plane mirror gives an object the appearance of 



rH 


r^ 


E F 


1 3 


LEC 


03J 


TION 


l/IOIT 







Fig. 7. — Lateral Inversion. 



optics. 1 5 

moving in the opposite direction to that in which the mirror 
is tilted. 

Spheric Mirrors. — A spheric mirror is a portion of a 
reflecting spheric surface ; its center of curvature is therefore 
the center of the sphere of which it is a part. Spheric mir- 
rors are of two kinds — concave and convex. 

Reflection from a Concave Mirror (Fig. 8). — Parallel 
rays are reflected from a concave mirror, and are brought to 
a focus in front of it. This point is called the principal focus 
(P.F.). The principal axis of a concave mirror is a straight 
line drawn from the mirror through the principal focus and 



Fig. 8. 

the center of curvature (i-i), and a secondary axis (2 r , 2' ', 
2', 2') is any other straight line passing from the mirror to 
the center of curvature (C.C.). Rays which diverge from 
any point beyond the principal focus are reflected con- 
vergently (G J). Rays which diverge from any point closer 
than the principal focus are reflected divergently (V V 7 ). 

Images Formed by a Concave Mirror. — To find the 
position of an image as formed by a concave mirror, two 
rays may be used : one drawn from a given point on the 
object to the mirror, and parallel to its principal axis, and 
reflected through the principal focus (P.F., Figs. 9 and 10); 
the other, the secondary axis, from the same point, passing 



1 6 REFRACTION AND HOW TO REFRACT. 

through the center of curvature. The place where the 
secondary axis and the reflected ray or their projections in- 
tersect gives the position of the image. Unlike the plane 
mirror, which produces images at all times and at all dis- 
tances, the concave mirror produces either an erect, virtual, 
and enlarged image, if an object is placed closer than its 
principal focus, or an enlarged inverted image if the object 
is between the principal focus and the center of curvature. 

By withdrawing the mirror in the former instance the 
erect image increases slightly in size, and in the latter the 
inverted image diminishes in size. At the principal focus 
there is no image formed. 



Fig. 9. 

Figure 9 shows an erect, virtual, and enlarged image of 
A R which is closer to the mirror than the principal focus. 
Parallel rays from A and R are reflected to the principal 
focus, P.F. Lines drawn from the center of curvature 
through A and R to the mirror are secondary axes ; these 
lines and those reflected to the principal focus do not inter- 
sect in front of the mirror, but if projected, will meet at a 
and r behind the mirror, forming a magnified image of 
A R. If the mirror is withdrawn from the object, the 
erect magnified image will increase in size, but at the prin- 
cipal focus no image will be formed, as the rays are reflected 
parallel. 



OPTICS. 



17 



Figure 10 shows a real inverted image of A R at a r ; 
A R situated beyond the principal focus. Lines drawn 
from A and R through C.C. are secondary axes. Parallel 
rays from A and R converge and cross at the principal 
focus (P.F.). 

Where D P and F E intersect the secondary axes, the in- 
verted image a r of A R is situated. When the object, as 
in this instance, is situated beyond the center of curvature, 
the image is smaller than the object. As the image and 
object are conjugate to each other, they are interchange- 
able, and in such a case the image would be larger than 
the object and inverted. This is always true when the 




Fig. 10. 



object is situated between the center of curvature and the 
principal focus. When an object is situated at the center 
of curvature, its image is equally distant and of the same 
size, but inverted. 

Tilting a concave mirror gives an object placed inside of 
its principal focus the appearance of moving as the mirror 
is tilted ; but if the object is situated beyond the principal 
focus, the object appears to move in the opposite direction. 

Reflection from a Convex Mirror. — All rays are re- 
flected divergently from a convex mirror, and parallel rays 
diverge as if they came from the principal focus situated 
behind the mirror at a distance equal to one-half its radius 



Io REFRACTION AND HOW TO REFRACT. 

of curvature. The principal focus of a convex mirror is 
therefore negative. The foci of convex mirrors are virtual. 

Images Formed by a Convex Mirror.— These are 
always virtual, erect, and smaller than the object. The 
closer the object, the larger the image ; and the more distant 
the object, the smaller the image. Tilting a convex mirror, 
the image does not appear to change position. 

In figure 1 1 parallel rays from the object A R are reflected 
from the mirror as if they came from the principal focus situ- 
ated at one-half the distance of the center of curvature, C.C. 
Lines drawn from the extremes of the object to C.C. are 




C.C 



Fig. ii. 

secondary axes, and the image is situated at the point of 
intersection of the secondary axes and the rays from the 
principal focus ; and as these meet behind the mirror, the 
image is virtual and erect. 

Refraction. — From the Latin refrangere, meaning "to 
bend back " — i. e. y to deviate from a straight course. Refrac- 
tion may be defined as the deviation which takes place in 
the direction of rays of light as they pass from one medium 
into another of different density. * 



* As ordinarily understood in ophthalmology, refraction has come to mean 
the optic condition of an eye in a state of repose or under the physiologic effect 
of a cycloplegic. 



OPTICS. 



19 



Two laws govern the refraction of rays of light : 

1. A ray of light passing from a rare into a denser 
medium is deviated or refracted toward the perpendicular. 

2. A ray of light passing from a dense into a rarer 
medium is deviated or refracted away from the perpen- 
dicular. 

Aside from these laws, there are other facts in regard 
to rays of light that should have consideration. A ray 
of light will continue its straight course through any 
number of different transparent media, no matter what their 
densities, so long as it forms right angles with the surface 



X 


/ 


^ ICE 


£> 


I FLINT GLASS 


A 


> CROWN " 


f 


J PLATE " 


i 








Fig. 12. 



Fig. 13. 



or surfaces. Such a ray is spoken of as the normal or per- 
pendicular ; such surfaces are plane, the surfaces and per- 
pendicular forming right angles. (See Fig. 12.) In any 
case of refraction the incident and refracted rays may be 
supposed to change places. 

Figure 13 shows the perpendicular (P P) to a piece of 
plate glass with plane surfaces. The ray in air incident at 
O on the surface S F is bent in the glass toward the per- 
pendicular, P P. The dotted line shows the direction the 
ray would have taken had it not been refracted. As the 
ray in the glass comes to the second surface at R, and 



20 REFRACTION AND HOW TO REFRACT. 

passes into a rarer medium, it is deviated from the perpen- 
dicular, P P. The ray now continues its original direction, 
but has been deviated from its course ; it has undergone 
lateral displacement. 

Critical Angle or Limiting Angle of Refraction. — This 
is the angle of incidence which just permits a ray of light 
in a dense medium to pass out into a rare medium. The 
size of the critical angle depends upon the index of refrac- 
tion of different substances. Figure 14 shows an electric 
light suspended in water. The ray from this light which 
forms an angle of 48 ° 35' with the surface of the water 




Fig. 14. — Critical Angle. 

will be refracted and pass out of the water, grazing its sur- 
face ; but those rays which form an angle greater than 
48 ° 35 r will not pass out of the water, but will be reflected 
back into it. The surface separating the two media be- 
comes a reflecting surface and acts as a plane mirror. 

The critical angle for crown glass is 40 49'. 

Index of Refraction. — By this is meant the relative 
density of a substance or the comparative length of time 
required for light to travel a definite distance in different 
substances. The absolute index of refraction is the 
density or refractive power of any substance as compared 



OPTICS. 



21 



with a vacuum. According to the first law of refraction, a 
ray of light passing from a rare into a dense medium is 
refracted toward the perpendicular ; in other words, the 
angle of refraction is smaller, under these circumstances, 
than the angle of incidence. In the study of the compara- 
tive density of any substance it 
will be seen that the angle of 
refraction is usually smaller the 
more dense the substance ; this 
is well illustrated in figures 15 
and 16. 

The greater the density, the 
slower the velocity or the more 

effort apparently for the wave or ray to pass through 
the substance. This is illustrated in figure 17, where a 
ray or wave of light is seen passing at right angles through 
different media. A ray passes through a vacuum without 
apparent resistance, but in its course through air it is 
slightly impeded, so that air has an index of refraction of 



v 




p 

1/ 


/ / 






/ / 





Fig. 15. 



Fig. 16. 



//■/.// 


I 






Ice 


Glass 


Diamond 


Vacuum 


Air- 



Fig. 17. 



1. 00029 -f- when compared with a vacuum ; but as this is so 
slight, air and a vacuum are considered as one for all pur- 
poses in refraction. To find the index of refraction of any 
substance as compared with a vacuum or air, it is neces- 
sary to divide the sine of the angle of incidence by the sine 
of the anfde of refraction. 



22 



REFRACTION AND HOW TO REFRACT. 



In figure 18 the angle of incidence P C I is the angle 
formed by the incident ray I with the perpendicular, P P. 
The angle of refraction P C R is the angle formed by the 
refracted ray with the perpendicular, P P. Drawing the 
circle P H P O around the point of incidence C, and then 

drawing the sines D 
P X and B F, perpen- 

'< diculars to the per- 

pendicular P P, divide 
the sine D X of the 
angle of incidence by 
the sine F B of the 
angle of refraction to 
obtain the index of 
refraction ; in this in- 
stance, water as com- 
pared with air. D X 
equaling 4 and F B 
equaling 3, then 4 di- 
vided by 3 will equal |-, or 
1.33 -f-> the index of refraction of water as compared with air. 
To find the index of refraction of a rare as compared 
with a dense substance, divide the sine of the angle of 
refraction by the sine of the angle of incidence — i. e. y air 
as compared with water would be ^, or 0.75. 




Indexes of Refraction. 

Air, 1.00029 

Water, 1. 333 

Cornea, L-3333 

Crown glass, 1.5 

Flint glass, 1.58 

Crystalline lens, nucleus, 1.43 

" " intermediate layer, 1. 41 

" " cortical layer, 1.39 



OPTICS. 



23 



A prism is a wedge-shaped portion of a refracting 
medium contained between two plane surfaces. The sides 
of a prism are the inclined surfaces. The apex is where 
the two plane surfaces meet. The base of the prism is the 
thickest part of the prism. The refracting angle is the 
angle at which the sides come together. 

Position of a Prism. — When a prism is placed in front 
of an eye, its position is indicated or described by the direc- 
tion in which its base is situated : base down means that the 
thick part of the prism is toward the cheek ; base up means 
that the thick part of the prism is toward the brow ; base 
in means that the thick part of the prism is toward the 





Fig. 19. 



Fig. 20. 



nose ; and base out means that the thick part of the prism 
is toward the temple. 

Prismatic Action. — Rays of light passing through a 
prism are always refracted toward the base of the prism. 
If an incident ray is perpendicular to the surface of a prism, 
there will be only one refraction, and that takes place at 
the point of emergence. The angle of incidence in this 
instance will equal the angle of the prism, and the maximum 
deviation takes place, as all the refraction is done at one 
surface. 

In figure 19 the incident ray (I) is perpendicular to the 
surface A B, and is not refracted until it comes to the sur- 



24 REFRACTION AND HOW TO REFRACT. 

face A C at E, when it is bent toward the base B C, all the 
refraction taking place at the surface A C. 

If an incident ray forms an angle other than a right 
angle with the first surface of the prism, then it will be re- 
fracted twice — as it enters and as it leaves the prism. 

In figure 20 X N is the perpendicular to the surface A B. 
The ray (I) incident at N is refracted toward this perpen- 
dicular and follows the course N E inside of the prism. 
On emergence it is refracted from the perpendicular E P of 
the surface A C, and in the direction of the base of the prism. 
If the incident ray (I) so falls upon the surface A B that 
the refracted ray (N E) is parallel to the base (B C), and 
the emergent ray is such that the angle of emergence 
equals the angle of incidence (I N X), as in this instance, then 
the angles of incidence and of emer- 
gence are equal, and the deviation is 
at a minimum, or the least possible. 

Angle of Deviation (Fig. 21). — 

This is the angle formed between the 

directions of the incident and emergent 

rays, and measures the total devia- 

FlG 2I tion. In all prisms of ten degrees 

or less the angle of deviation is equal 

to half the angle of the prism, but in prisms of more than 

ten degrees the angle of deviation increases. 

Summary. — Prisms do not cause rays of light to con- 
verge or to diverge ; rays that are parallel before refraction 
are parallel after refraction. Therefore, prisms do not form 
images ; prisms have no foci. 

Effect of a Prism. — An object viewed through a prism 
has the appearance of being displaced, and in a direction 
opposite to the base — i. e., toward the apex. 

Rays from the object (X, Fig. 22) strike the prism at C, 




OPTICS. 



25 




Fig. 22. 



undergo double refraction, and, falling upon the retina of 
the eye, are projected back in the direction in which they 
were received, and the apparent po- 
sition of X is changed to X', away 
from the base of the prism and 
toward the apex. 

Numbering of Prisms. — Form- 
erly, prisms were numbered by their 
refracting angles ; now, however, 
two other methods are in use : 

Dennet's method, known as the centrad ; and Prentice's 
method, known as the prism- diopter. 

Dennett's Method (Fig. 23). — The unit, or centrad (ab- 
breviated T ), is a prism that will deviate a ray of light the 
Y^-q part of the arc of the radian. This is calculated as 
follows : As much of the circumference of a circle is taken 
as will equal the length of its radius of curvature ; this is 
called the arc of the radian, and equals 57.295 degrees. 
The arc of the radian is then divided into 100 parts. A 





prism, base down, at the center of curvature that will devi- 
ate a ray of light downward just y^- part of the arc of the 
radian is a one centrad, and equals y^-g- of 57.295 degrees, 
or 0.57295 of a degree. 
3 



26 



REFRACTION AND HOW TO REFRACT. 



Ten centrads will deviate a ray of light ten times as 
much as one centrad, or 10 X 0.57295 — 5.7295 degrees, 
etc. 

Prentice's Method (Fig. 24). — The unit, or prism-diopter 

(abbreviated P.D.,or A), is a prism 

98765432 10 that will deviate a ray of light 

just 1 cm. for each meter of dis- 
tance — that is, the yj-g- part of 
the radius measured on the tan- 
gent. The deviation always be- 
ing 1 cm. for each meter of dis- 
tance, 1 P. D. will deviate a ray of 
light 2 cm. for 2 meters of dis- 
tance ; 3 cm. for 3 meters, etc. 
The comparative values of cen- 
trads and prism-diopters is quite 
uniform up to 20, but above 20 
the centrad is the stronger. 

Neutralization of Prisms. — 
Knowing that rays of light are de- 
viated by centrads and prism- 
diopters up to 20, in the ratio of 
I cm. for each meter of distance, 
then to find the numeric strength 
of any prism all that is necessary 
is to hold the prism over a series 
of numbered parallel lines, sepa- 
Fig. 25. rated by an interval of 1 cm. or 

fraction thereof, and note the 
amount of displacement. For example, figure 25 shows a 
series of vertical lines ^ of a cm. apart, and numbered from 
o to 9 ; an X is placed at the foot of the o line. Holding a 
prism, base to the right, at a distance of ^ of a meter (as 



98765432 



x 
I 













> 


: 



OPTICS. 



27 



the lines are ^3 of a cm. apart) and looking through the prism 
at the X on the o line, it will be seen that the X has been 
displaced to the line to the left corresponding to the number 
of centrads or prism-diopters in the prism ; in this instance 
three. 

Table Showing the Equivalence of Centrads in Prism-diopters 

and in Degrees of the Refracting Angle (Index of 

Refraction 1.54). 



Centrads. 


Prism-diopters. 


Refracting Angle. 


f 


I. 


I°.oo 


2. 


2.0001 


2°. 12 


3- 


3.0013 


3 °.i8 


4 


4.0028 


4°- 23 


5. 


5-°°45 


5°-28 


6. 


6.0063 


6°. 3 2 


7- 


7.0115 


7°-35 


8. 


8.0172 


8°. 3 8 


9- 


9.0244 


9°-39 


10. 


10.033 


io°-39 


11. 


11.044 


ii°. 3 7 


12. 


12.057 


12 . 34 


*3- 


i3-o74 


I 3 °.2 9 


14. 


14.092 


I 4 °.23 


15. 


15. 114 


I5°.i6 


16. 


16.138 


i6°.o8 


17. 


17.164 


i6°.98 


18. 


18.196 


i 7 °.8 5 


19. 


19.230 


i8°.68 


20. 


20.270 


i9°-45 


25- 


25-55 


23°-43 


30- 


3°-934 


26 . 81 


35- 


36.50 


29°.72 


40. 


42.28 


32°. 18 


45- 


48.30 


34°. 20 


5o- 


54-5H 


35°-94 


60. 


68.43 


38°-3i 


70. 


84.22 


39°-73 


80. 


102.96 


4O .29 


90. 


126.01 


40°.49 


100. 


155-75 


39°. 14 



Or a prism may be neutralized by placing another prism 
in apposition to it, with their bases opposite, so that in look- 



28 REFRACTION AND HOW TO REFRACT. 

ing through the two prisms at a straight line, no matter at 
what distance, the straight line will continue to make one 
straight line through the prisms ; the strength of the neu- 
tralizing prism will equal the strength of the prism being 
neutralized. 

Uses of Prisms. — i. To detect malingerers who profess 
monocular blindness so as to obtain damages for supposed 
injuries, or who wish to escape war service, or those cases 
of hysteric blindness wishing to create sympathy. This 
test or use of a prism is known as the diplopia test, and is 
practised as follows : A seven P. D., base up or down, with 
a blank are placed in the trial-frame corresponding to the 
" blind " eye ; nothing is placed in front of the seeing eye ; 
the trial-frame, thus armed (without the patient seeing what 
is being done), is placed on the patient's face and he is in- 
structed to read the card of test-letters on the wall across 
the room. While he is thus busy reading, and purposely 
contradicted by the surgeon, so as to get his mind from his 
condition, the surgeon suddenly removes the blank from 
the " blind " eye. The patient exclaiming that he sees two 
cards and two of all the letters proves the deception. 

2. Occasionally, to counteract the effects of strabismus, 
or diplopia due to a paralysis of one or more of the extra- 
ocular muscles. For example : A patient looking at a 
point of light focused on the macula (M) of the left eye 
(L), the right eye being turned in toward the nose, receives 
the rays upon the retina to the nasal side of the macula, 
and hence projects the rays outward to the right, giving a 
false image to the right side ; a prism of sufficient strength 
is then placed with its base toward the temple (base out) 
over the right eye, so that the rays from the light may fall 
upon the macula (M), and the diplopia will be corrected 
(See Fig. 26.) 



OPTICS. 



2 9 



3. To test the strength of the extra-ocular muscles : A 
patient looking with both eyes at a distant point of light 
is made to see one light just above another by placing a 
3 P. D., base down or up, before either eye, and if a 2^ 
P. D. did not produce diplopia when similarly placed, the 
strength of his vertical recti is then represented by 2j4 
P. D. The strength of the prism placed base in which, 




Fig. 26. 



if increased, would produce diplopia is the strength of the 
externi ; and the strength of the prism or prisms placed 
base outward which, if increased, would produce diplopia 
is the strength of the interni. 

4. For exercise of weak muscles. (See p. 191.) 
Lenses. — A lens is a portion of transparent substance 
(usually of glass) having one or both surfaces curved. 
There are two kinds of lenses — spheric and cylindric. 



30 REFRACTION AND HOW TO REFRACT. 

Spheric Lenses. — Abbreviated S. or sph. Spheric 
lenses are so named because their curved surfaces are sec- 
tions of spheres. A spheric lens is one which refracts 
rays of light equally in all meridians or planes. Spheric 
lenses are of two kinds — convex and concave. 

A convex spheric lens is thick at the center and thin 
at the edge, (Figs. 27, 28, 29.) The following are synony- 
mous terms for a convex lens : (1) Plus ; (2) positive ; (3) 
collective ; (4) magnifying. A convex lens is denoted by 
the sign of plus ( + )• 

Varieties or Kinds of Convex Lenses. — 

I. Planoconvex, meaning one surface flat and the other 

convex. It is a section of a 
sphere. (See Fig. 27.) 

2. Biconvex, also called con- 
vexoconvex or bispheric, for the 
reason that it is equal to two 
planoconvex lenses with their 
plane surfaces together. (Fig. 

Fig. 27. Fig. 28. Fig. 29. 28.) 

3. Concavoconvex. This lens 
has one surface concave and the other convex, the convex 
surface having the shortest radius of curvature. (Fig. 29.) 
The following are synonymous terms for a concavoconvex 
lens : (1) Periscopic ; (2) convex meniscus ; (3) converging 
meniscus (meniscus meaning a small moon). (See Fig. 29.) 
A periscopic lens enlarges the field of vision, and is of 
especial service in presbyopia. 

A Concave Spheric Lens. — Such a lens is thick at 
the edge and thin at the center. (Figs. 30, 31, 32.) The 
following are synonymous terms for a concave lens: (1) 
Minus ; (2) negative ; (3) dispersive ; (4) minifying. A 
concave lens is denoted by the sign of minus ( — ). 



OPTICS. 



31 



Varieties or Kinds of Concave Lenses. — 

1. Planoconcave, meaning one surface flat and the other 
concave. (Fig. 30.) 

2. Biconcave, also called concavoconcave or biconcave 
spheric, for the reason that it is 

equal to two planoconcave lenses 
with their plane surfaces to- 
gether. (Fig. 31.) 

3. Convexoconcave. This lens 
has one surface convex and the 
other concave, the concave sur- 
face having the shortest radius 
of curvature. (Fig. 32.) The 

following are synonymous terms for a concavoconvex lens 
(1) Concave meniscus ; (2) diverging meniscus ; (3) peri- 
scopic. 



Fig. 30. Fig. 31. Fig. 32. 





Fig 33- 



Fig. 34. 



A spheric lens may be considered as made up of a series 
of prisms which gradually increase in strength from the 
center to the periphery, no matter whether the lens be con- 
cave or convex. 



32 REFRACTION AND HOW TO REFRACT. 

In the convex sphere the bases of the prisms are toward 
the center of the lens, whereas in the concave the bases of 
the prisms are toward the edge. (See Figs. 33, 34.) 

Knowing that a prism refracts rays of light toward its 
base, it may be stated as a rule that every lens bends rays 
of light more sharply as the periphery is approached — i. e. f 
at the periphery the strongest prismatic effect takes place. 

Lens Action. — As a ray of light will travel in a straight 
line so long as it continues to form right angles with sur- 
faces, then the ray A in figure 35 passes through the bicon- 
vex lens unrefracted, or without any deviation from its 
course whatsoever, for at its points of entrance and emer- 




Fig. 35. 

gence the surfaces of the lens are plane to each other. This 
ray is called the axial ray, and the line joining the centers 
of curvature of the two surfaces is called the principal axis. 
The axis of a planoconvex or planoconcave lens is the line 
drawn through the center of curvature perpendicular to the 
plane surface. 

The ray B in figure 35, though parallel to the ray A, 
forms a small angle of incidence, and must, therefore, be 
refracted toward the perpendicular to the surfaces of the 
lens, and, passing through the lens, will meet the axial ray 
at P.F. The rays C, D, and E, also parallel to A and B, 
form progressively larger angles with the surface of the lens, 



OPTICS. 



33 



and finally meet the axial ray at P.F. It will be seen at 
once that the rays all meet at P.F., showing the progres- 
sively stronger prismatic action that takes place as the per- 
iphery of the lens is approached. 

In figure 36 we have similar rays, A, B, C, D, and E, 
passing through a con- 



cave 



lens. 



The axial 




V-?. 



Fig. 36. 



ray A passes through 
the centers of curvature 
un refracted, but the rays 

B, C, D, and E are pro- 
gressively refracted, more 
and more as the periph- 
ery is approached. The 

ray E in each instance is refracted the most. 

The action of a convex lens is similar to that of a concave 
mirror, and the action of a concave lens is similar to that of 
a convex mirror. 

Principal Focus. — The principal focus of a lens may be 
defined (1) as the point where parallel rays, after refrac- 
tion, come together on the axial ray ; or (2) as the shortest 
focus ; or (3) as the focal point for parallel rays. 

Focal Length. — This is the distance measured from the 
optic center to the principal focus. The principal focus 
of an equally biconvex or biconcave lens of crown glass is 
situated at about the center of curvature for either surface 
of the lens. A lens has two principal foci, an anterior and 
a posterior, according to the direction from which the par- 
allel rays come, or as to which radius of curvature is re- 
ferred to. (Seep. 60.) Figure 35 shows parallel rays, B, 

C, D, and E, passing through a convex lens and coming 
to a focus on the axial ray (A) at P. F. ; and as the path of 
a ray passing from one point to another is the same, no 



34 REFRACTION AND HOW TO REFRACT. 

matter what its direction, then if a point of light be placed 
at the principal focus of a lens, its rays will be parallel after 
passing back through the lens. This is equivalent to what 
takes place in the standard or emmetropic eye. An eye, in 
other words, which has its fovea situated just at the princi- 
pal focus of its dioptric media, such an eye in a state of rest 
receives parallel rays exactly at a focus upon its fovea, and 
therefore is in a condition to project parallel rays outward. 
Conjugate Foci. — Conjugate meaning "yoked to- 
gether." The point from which rays of light diverge 
(called the radiant) and the point to which they converge 
(called the focus) are conjugate foci or points. For in- 
stance, in figure 37 the rays diverging from A and passing 




Fig. 37. 

through the lens converge to the point B ; then the points 
A and B are conjugate foci. They are interchangeable, for 
if rays diverged from B, they would follow the same path 
back again and meet at A. The path of the ray C C is 
the same whether it passes from A to B or from B to A : 
there is no difference. It is by the affinity of these points 
for each other, with respect to their positions, that they are 
called conjugate. 

The conjugate foci are equal when the point of diver- 
gence is at twice the distance of the principal focus. The 
equivalent to conjugate foci is found in the long or myopic 
eye ; an eye, in other words, which has its fovea situated 
further back than the principal focus of its dioptric media, 
the result being that rays of light from the fovea of such an 



optics. 3 5 

eye would be projected convergently after passing out of the 
eye, and would meet at some point inside of infinity. In 
other words, only those rays which have diverged from some 
point inside of six meters will focus upon the fovea of this 
long eye. The fovea of the myopic eye represents a con- 
jugate focus. A myopic eye is in a condition to receive 
divergent rays ©f light at a focus on its retina and to emit 
convergent rays. 

Ordinary Foci. — When rays of light diverge from some 
point inside of infinity (six meters) they will be brought to 
a focus at some point on the other side of a convex lens, 
beyond its principal focus ; this point is called an ordinary 
focus. A lens may have many foci, but only two principal 




Fig. 38. 

foci. The further away from a lens the divergent rays pro- 
ceed, the nearer to the principal focus on the other side of 
the lens will they converge. As the divergent rays are 
brought closer to the lens they reach a point where they 
will not focus, but will pass parallel after refraction. This 
point is the principal focus. (See Fig. 38.) A lens, there- 
fore, has as many foci as there are imaginary points on the 
axial ray between the principal focus and infinity. 

\\ hen rays of light diverge from some point closer to a 
lens than its principal focus, they do not converge, but, 
after refraction, continue divergently; their focus now is 
negative or virtual, and is found by projecting these diver- 
gent rays back upon themselves to a point on the same 



36 



REFRACTION AND HOW TO REFRACT. 



side of the lens from which they appeared to come. (See 
Fig- 39-) 

This is the equivalent of what takes place in a short or 
hyperopic eye, an eye which has its macula closer to its 
dioptric media than its principal focus. In a state of rest 




Fig. 39. 

the fovea of such an eye would project outward divergent 
rays, and would be in a position to receive only convergent 
rays of light at a focus upon its fovea. 

Secondary Axes. — In the study of the direction of a ray 
of light passing through a dense medium with plane sur- 




Fig. 40. 



faces, it was found that it underwent lateral displacement 
(see Fig. 13), and so in lenses there is a place where rays 
undergo lateral displacement. Figure 40 shows a convex 
lens of considerable thickness, and on each side is drawn a 
radius of curvature (C C). The ray indicated by the arrow 



optics. 37 

passed through the two surfaces, has undergone lateral 
displacement, but continues in its original direction ; such 
rays are called secondary rays or axes. The incident ray 
is projected toward N 1 in the lens on the axial ray, and 
the emergent ray, if projected backward, would meet the 
axial ray at N 2 . These points on the axial ray are such 
that a ray directed to one before refraction, is directed to 
the other after refraction. The points N 1 and N 2 are 
spoken of as nodal points. Every lens, therefore, has two 
nodal points, but in thin lenses the deviation of the second- 
ary rays is so slight that, for all practical purposes, only 




Fig. 41. 

one nodal point is recognized. It is spoken of as the optic 
center. When writing prescriptions for glasses, this point of 
having the lenses or glasses made as thin as possible, must 
be borne in mind. 

Optic Center. — This term is used synonymously with 
nodal point, and is the point where the secondary rays (s. a. in 
Fig. 41) cross the axial ray. It is not always the geometric 
center. Rays of light crossing the optic center in thin lenses 
are not considered as undergoing refraction. (See Fig. 41.) 

Action of Concave Lenses. — Rays of light passing 
through a concave lens, no matter from what distance, are 
always refracted divergently, and its focus is, therefore, 
always negative or virtual, and is found by projecting these 



38 



REFRACTION AND HOW TO REFRACT. 



divergent rays backward in the direction From which they 
appear to come until they meet at a point on the axial ray. 
The principal focus and conjugate foci of concave lenses 
are found in the same way as in convex lenses. (See Figs. 
36. 44-) 

Images Formed by Lenses. — An image formed by a 
lens is composed of foci, each one of which corresponds 
to a point in the object. Images are of two kinds — real and 
virtual. 

A Real Image. — This is an image formed by the actual 
meeting of rays ; such images can always be projected on 
to a screen. 

A Virtual Image. — This is one that is formed by the 
prolongation backward of rays of light to a point. 




Fig. 42 



To find the position and size of an image it is necessary 
to obtain the conjugate foci of the extremes of the object, 
as the image of an object is equal to the sum of its inter- 
mediate points. Only two rays are required for this pur- 
pose, one parallel to the axial ray, and one secondary ray 
passing through the optic center ; the image of the extreme 
point of the object will be located at the point of inter- 
section of these rays. In figure 42 A B is an object in 
front of a convex lens, o is the optic center and P. s F. 
the principal focus. A ray drawn from A parallel to the 
axial ray o, and a secondary ray from the same point drawn 



optics. 39 

through the optic center, will give at their point of inter- 
section the conjugate focus of the luminous point A, which 
will be at A'. In the same way the conjugate focus of B 
and points intermediate in the object may be obtained. A' 
B' is a real inverted image of A B ; the size of the image 
of A B depends upon the distance of the object from 
the lens. The relative sizes of image and object are as 
their respective distances from the optic center of the lens. 
For example, if an object ten millimeters high is three 
meters (3000 mm.) from the optic center of a lens, and its 
image is sixty millimeters from the lens, the image will be 
3"f oT or oV °f tne s * ze °^ *- ne object ; that is, the image will 
be -Jj of ten millimeters (the height of the object) — namely, 
i of a millimeter hi^h. 

As conjugate foci are interchangeable, then in figure 42 
if A' B' was the object, the image A B would be the image 
of A' B', and, therefore, larger than the object. 

Three facts should be borne in mind in the study of real 
images formed by a convex lens : 

1. The object and image are interchangeable. 

2. The object and the real image are on opposite sides of 
the lens, and, 

3. As the rays which pass through the optic center 
cross each other at this point, the real image must be in- 
verted. 

Rays of light from an object situated at the distance of 
the principal focus would proceed parallel after refraction, 
and no image of the object would be obtained. 

If an object is situated just beyond the principal focus, 
then the image would be larger than the object, real and 
inverted. (See Fig. 42, reversing image for object.) 

If an object is situated at twice the distance of the prin- 
cipal focus, then its image would be of the same size, real, 



4Q 



REFRACTION AND HOW TO REFRACT. 



inverted, and at a corresponding distance, as these conju- 
gate foci are equal. 

If an object is situated at a greater distance than twice 
the principal focus, and nearer than infinity, its image will 
be real, inverted, and smaller than the object. 




Fig. 43. 

Rays of light from an object situated closer to a lens 
than its principal focus would be divergent after refraction, 
and could only meet by being projected backward ; the 
image would, therefore, be larger than the object, erect, and 




Fig. 44. 



virtual. Such an image is only seen by looking through 
the lens ; the lens in this instance being a magnifying 
glass. (Fig. 43.) 

Images Formed by Concave Lenses. — These images 
are always erect, virtual, and smaller than the object. (See 
Fig. 44.) A concave lens is, therefore, a minifying lens. 



OPTICS. 41 

Parallel rays from the extremes of the object A R form the 
divergent ray A' and R' after refraction. Secondary rays 
pass through the optic center o unrefracted, A" and R". 
At the points of intersection where these rays meet after 
being projected backward, the image of A R is found, 
erect, virtual, and diminished in size. This image is only 
seen by looking through the lens. 

Numeration of Lenses. — Formerly, lenses were num- 
bered according to their radii of curvature in Paris inches 
(27.07 mm.). The unit was a lens that focused parallel 
rays of light at the distance of one English inch (25.4 mm.) 
from its optic center. 

As lenses for purposes of refraction were never as strong 
as the unit, they were numbered by fractions, thus showing 
their relative strength as compared to this unit ; for instance, 
a lens that was one-fourth the strength of the unit was 
expressed by the fraction %, or a lens that was one- 
sixteenth the strength of the unitw^as expressed as ^ etc., 
the denominator of the fraction indicating the focal length 
of the lens in Paris inches. 

There are three objections to this nomenclature : (1) 
The difference in length of the inch in different countries ; 
(2) the inconvenience of adding two or more lenses num- 
bered in fractions with different denominators — y^- -f x§" 
-f T V; (3) the want of uniform intervals between num- 
bers. 

In the new nomenclature, and the one that is now quite 
universal, known as the metric or dioptric system (diopter, 
abbreviated D.), a lens has been taken as the unit which 
has its principal focus at one meter distance (39.37 English 
inches), commonly recognized as 40 inches. 

Lenses in the dioptric system are numbered according 
to their refractive power and not according to their radii of 
4 



42 REFRACTION AND HOW TO REFRACT. 

curvature. The strength or refractive power of a dioptric 
lens is, therefore, the inverse of its focal distance. To find 
the focal distance of any dioptric lens in inches or centi- 
meters, the number of diopters expressed must be divided 
into the unit of 40 inches or 100 cm. For example, a 
2 D. lens has a focal distance of 40 -f- 2 equals 20 inches ; 
or 100 cm. ~- 2 equals 50 cm. A +4 D. has a focal dis- 
tance of 40 -f- 4, equaling 10 inches, or 100 -r- 4, equaling 25 
cm. Lenses that have a refractive power less than the unit 
are not expressed in the form of fractions, but in the form of 
decimals ; for example, a lens which is only one -fourth, one- 
half, or three-fourths the strength of the unit is written 0.25, 
0.50, 0.75, respectively, and their focal distances are found 
in the same way as in dealing with units : 0.25 D. has a 
focal distance of 40 -s- 0.25 or 100-^-0.25, equaling 160 
inches or 400 cm. ; 0.50 D. has a focal length of 40 -*- 
0.50 or 100 -v- 0.50, equaling 80 inches or 200 cm. ; 0.75 
D. has a focal length of 40 -f- 0.75 or 100 -s- 0.75 
equaling 53 inches or 133 cm. Unfortunately, 0.25 D., 
0.50 D., and 0.75 D. are frequently spoken of as twenty- 
five, fifty, and seventy-five, which occasionally leads to 
confusion in the consideration of the strength and focal dis- 
tance. The student should learn as soon as possible to 
change the old nomenclature into the new, as he will have 
to make these changes in reading other text-books. 

To change the old " focal length " or inch system of 
numbering lenses into diopters, divide the unit (40 in.) by 
the denominator of the fraction, and the result will be an 
approximation in diopters ; for example, y^- equals j^ 
or 4 D. ; -Jq- equals |-§- or 2 D. The following table, 
from Landolt, gives the equivalents in the old and new 
systems : 



OPTICS. 



43 



OLD SYSTEM. 


NEW SYSTEM. 


I. 


II. 


III. 


IV. 


V. 


VI. 


VII. 


VIII. 


Xo. 








No. 






No. 


of the 


Focal 


Focal 




of the 


Focal 


Focal 


Corres- 


Lens, 


Distance 


Distance 


Equiva- 


Lens, 


Distance 


Distance 


ponding 


Old 


in English 


in Milli- 


lent in 


New 


in Milli- 


in English 


of the Old 


System. 
72 


Inches. 


meters. 


Diopters. 


System. 


meters. 


Inches. 


System. 


67.9 


1724 


0.5S 


0.25 


4000 


15748 


166.94 


60 


56.6 


1437 


0.695 


0.5 


2000 


78 74 


8346 


4S 


45-3 


1150 


0.87 


0-75 


1333 


52.5 


55 63 


42 


39-6 


1005 


0.99 


1 


1000 


39 37 


41-73 


36 


34 


863 


1. 16 


125 


800 


31-5 


33 39 


30 


28.3 


718 


i-39 


i-5 


666 


26.22 


27 79 


24 


22.6 


574 


1.74 


J-75 


571 


22.48 


23-83 


20 


18.8 


477 


2.09 


2 


500 


19.69 


20.87 


18 


17 


431 


2.31 


2.25 


444 


17.48 


1853 


16 


15 


38i 


2.6 


2-5 


400 


15-75 


16.69 


15 


14.1 


358 


2.79 


3 


333 


13 17 


13-9 


14 


13-2 


335 


2.98 


3-5 


286 


11 26 


11.94 


13 


12.2 


312 


3.20 


4 


250 


9.84 


10.43 


12 


11. 2 


287 


3-48 


4-5 


222 


8.74 


9 26 


11 


10.3 


261 


3-82 


5 


200 


7.87 


8.35 


10 


9-4 


239 


4.18 


5-5 


182 


7.16 


76 


9 


8-5 


216 


4-63 


6 


166 


6-54 


6 93 


8 


7-5 


190 


5-25 


7 


143 


563 


5-97 


I, , 


6.6 


167 


5-96 


8 


125 


4.92 


5.22 


6% 


6.13 


155 


6.42 


9 


in 


4-37 


4-63 


6 


5-6 


142 


7.0 


10 


100 


3-94 


4 17 


5% 


52 


132 


7-57 


11 


9i 


3-58 


3-8 


5 


4-7 


119 


8.4 


12 


83 


3-27 


3-46 


4^ 


4.2 


106 


9-4 


13 


77 


3-03 


3-21 


4 


3-8 


96 


10.4 


H 


7i 


2.8 


2.96 


3^ 


3-3 


84 


11.9 


15 


67 


2.64 


2.8 


35i 


3-1 


79 


12.7 


16 


62 


244 


2-59 


3 , 


2.8 


7i 


14.0 


17 


59 


2.32 


2.46 


2 H 


2.6 


66 


i5-i 


18 


55 


2 17 


2.29 


2% 


2.36 


60 


17.7 


20 


50 


1.97 


2.09 


2* 


2.1 


53 


18.7 










2 


1.88 


48 


20.94 











Cylindric Lenses. — Abbreviated cyl., c, or C. A cylin- 
dric lens, usually called a " cylinder," receives its name from 
being a segment of a cylinder parallel to its axis. (See 
Fig. 45.) Occasionally cylinders are made with both sur- 
faces curved, and are then equivalent to two planocylinders 
with their plane surfaces together. A cylinder may be 
defined as a lens which refracts rays of light opposite to its 
axis. This definition should be carefully borne in mind in 
contradistinction to a spheric lens, which refracts rays of 
light equally in all meridians. A cylindric lens has no one 



44 



REFRACTION AND HOW TO REFRACT. 



common focus or focal point, but a line of foci, which is 
parallel to its axis. 

Axis of a Cylinder. — That dimension of a cylindric lens 
which is parallel to the axis of the original cylinder of 






Fig. 45. 



Fig. 46. 



Fig. 47. 



which it is a part is spoken of as the axis, and is indicated 
on the lens of the trial-case by a short diamond scratch on 
the lens at its periphery, or by having a small portion of 




Fig. 48. 



its surface corresponding to the axis ground at the edges, 
or it may be marked in both ways. (See Fig. 47.) Cylin- 
ders are of two kinds — convex and concave. (Figs. 45, 46.) 
Cylinder Action. — A convex cylinder converges parallel 



OPTICS. 



45 




rays of light so that after refraction they are brought into 
a straight line which corresponds to the axis of the cylin- 
der ; for instance, a -f- 5 cyl. will converge parallel rays so 
that they come together in a straight line at the dis- 
tance of eight inches, 
or twenty centimeters, 
and this straight line 
will be parallel to the F«= 
axis of the cylinder. 
(Fig. 48.) 

A concave cylin- 
der diverges rays of Fie 4 . 
light opposite to its 

axis, as if they had diverged from a straight line on the 
opposite side of the lens. (Fig. 49.) 

Spherocylinders. — A spherocylinder is a combination 
of a sphere and a cylinder, and is therefore a lens which has 
one surface ground with a spheric curve and the other sur- 
face cylindric. A spherocylindric lens is also spoken of as 
an astigmatic lens. (See Fig. 100.) A spherocylindric lens 
is one which has two focal planes. Spherocylinders have 
different curves : the spheric curve may be convex, with the 
cylindric surface convex ; or the spheric surface may be 
concave, with the cylindric surface concave ; or the spheric 
surface may be convex, with the cylindric surface concave ; 
or the spheric surface may be concave, with the cylindric 
surface convex. 

The Trial-case (see Fig. 50). — This case contains pairs 
of plus and minus spheres and pairs of plus and minus 
cylinders ; also prisms numbered from ^ or y 2 to 20 A. 
The spheres are numbered in intervals of 0.12 up to 2 S.; 
and from 2 S. up to 5 S. the interval is 0.25 S.; and from 
5 S. to 8 S. the interval is 0.50 S.; and from 8 S. to 22 S. 



4 6 



REFRACTION AND HOW TO REFRACT. 



the interval is I S. The cylinders have similar intervals, 
but seldom go higher than 6 or 8 cyl. 

The trial-case also contains a trial-frame, which is used 
to place lenses in front of the patient's eyes. (See Fig. 51.) 
The eye -pieces of such a frame are numbered on the 
periphery in degrees of half a circle, so that the axis of a 
cylinder can be seen during refraction. The left of the 




Fig. 50. 



horizontal line in each eye-piece is recognized as the start- 
ing-place, or zero (o), and the degrees are marked from 
left to right on the lower half, counting around to the 
horizontal meridian, which at the right hand is numbered 
180; this horizontal meridian is, therefore, spoken of as 
horizontal, zero (o), or 180 degrees. The meridian midway 
between zero and 180 is spoken of as vertical, or 90 degrees. 
In some countries the meridians are differently num- 



OPTICS. 



47 



bered (see Fig. 52); for example, the vertical meridian is 
called zero, and the degrees are marked on each side of 
zero up to 90 degrees. Only the upper half of the eye -piece 
is thus numbered, so that when a cylinder has the upper end 
of its axis inclined toward the nose, the record would be so 
many degrees of inclination to the nasal side ; or if the upper 
end of the cylinder was inclined toward the temple, the 
record would be so many degrees to the temporal 'side. 
For example, in the right eye 1 5 degrees nasal would 




Fig. 51. 

mean axis 75 on the ordinary trial-frame, and 15 degrees 
temporal would mean 105 degrees. 

The trial-case also contains other accessories, such as 
blanks or blinders, a stenopeic slit, pin-hole disc, etc., all of 
which are referred to in the text. 

Combination of Lenses.— The sign of combination 
is ^~\ 

Combining Spheres. — Any number of spheric lenses 
placed with their optic centers over each other, and sur- 



4 8 



REFRACTION AND HOW TO REFRACT. 



faces together, will equal one lens the value of their sum : 
for example, -f2S.Q-f-1S.Q-f3 S. will equal -f 6 S.; 
or a — 2 S. Q — 1 S. Q — 3 S. will equal a — 6 S. 

If a plus and minus sphere, each of the same strength, be 
placed with their optic centers together, the refraction will 
be nothing, for the one will neutralize the effect of the other ; 
for instance, -f 4 S. and — 4 S. will be equivalent to a piece 
of plane glass, as the — 4 S. will diverge rays of light as 
much as the +4 S. will converge them, and the result 




Fig. 52. 



is, rays of light parallel before refraction are parallel after 
passing through such a combination. If, however, a plus 
and a minus sphere of different strengths are placed together, 
the value of the resulting lens will equal their difference, in 
favor of the higher number; for instance, +4 S. and — 2 
S. will equal a -f 2 S., the — 2 S. neutralizing 2 S. of the 
-f 4 S., leaving -f 2 S. 

Combining Cylindric Lenses. — Any number of cylin- 
dric lenses placed together, with their axes in the same 



optics. 49 

meridian, are equal to a cylinder the value of their sum ; 
for example : -f-2 cyl. axis 90 degrees and -j-3 cyl. axis 90 
degrees will equal a + 5 cyl. axis 90 degrees ; or — 2 cyl. 
axis 180 degrees and — 3 cyl. axis 180 degrees will equal 
a — 5 cyl. axis 180 degrees ; or — 2 cyl. axis 180 degrees 
and -pi cyl. axis 180 degrees will equal a — 1 cyl. axis 
180 degrees. 

As a cylinder refracts rays of light only in the meridian 
opposite to its axis, this opposite meridian can always be 
found by the following simple rule : 

" Add go when the give?i axis is go or less than go, and 
subtract go when the given axis is more than go." 

For example : + 3 c yl- ax i s 9° refracts rays of light in 
the 180 degree meridian (90 + 90=180); or +3 cyl. 
axis 75 refracts rays of light in the 165 meridian (75+90 
= 165). A — 3 cyl. axis 135 refracts rays of light in the 
45 meridian (135 less 90 =45). A — 2 cyl. axis 180 re- 
fracts rays of light in the 90 meridian (180 less 90 = 90). 

Combining two cylinders of the same strength and 
same denomination, with their axes at right angles to 
each .other, will equal a sphere of the same strength 
and same denomination. For instance, +3 cyl. axis 90 
and —3 cyl. axis 180, placed together, will equal a +3 S. 
— i. e. y the +3 cyl. at axis 90 will converge parallel rays in 
the 180 meridian, while the +3 cyl. axis 180 will converge 
parallel rays in the 90 meridian, producing a principal 
focus ; therefore any sphere is also equal to two cylinders 
of its same strength and same denomination with their 
axes at right angles to each other. 

Combining cylinders of different strength, but of the 

same denomination, with their axes at right angles to 

each other, such a combination will equal a sphere and 

a cylinder of the same denomination. For example : 

5 



50 



REFRACTION AND HOW TO REFRACT. 



-\-2 cyl. axis 75 O +3 cyl. axis 165 will equal +2 S. C 
+ 1 cyl. axis 165. The -f- 2 cyl. axis 75 takes +2 of the 
-f 3 cyl. axis 165 and makes a -\-2 S., leaving -f- 1 cyl. axis 
at 165 ; the result is then -j-2 S. O + 1 cyl. axis 165. 

Or — 3.50 cyl. axis 15 O — 4.50 cyl. axis 105, will 
equal — 3.50 S. O — 1 cyl. axis 105. The — 3.50 axis 15 
takes — 3.50 of the — 4.50 and makes a — 3.50 S., 
leaving — 1 cyl. axis 105 ; this — 1 cyl. axis 105 is now 
joined to the — 3.50 sphere, making — 3.50 S. O — I cyl. 
axis 105. 

Combining a sphere and a cylinder of the same 

strength, but of differ- 
ent denomination, will 
equal a cylinder of the 
opposite sign and opposite 
axis from the cylinder 
given. For example : -j- 1 
sphere 3 — l cyl. axis 1 80 
will equal + 1 cyl. axis 90. 
The -f- 1 S. equals two -f 1 
cylinders, one at axis 90 
and one at axis 180, and 
the — 1 cyl. at axis 180 is 
neutralized by the -f 1 cyl. 
at the same axis, leaving 
the -f- 1 cyl. axis 90. This may be better understood by the 
diagram (Fig. 49). 

Or — 3 S. O + 3 cyl. axis 90 equals — 3 cyl. axis 180. 
The — 3 S. is equal to two — 3 cylinders, one at axis 90 
and one at axis 180 ; the one at axis 90 is neutralized by 
the 4-3 cyl. at the same axis, leaving — 3 cyl. axis 180. 

Combining a Sphere with a Weaker Cylinder of Dif- 
ferent Denomination. — Such a combination should be 




+ 1.00 



Fig. 53- 



OPTICS. 5 1 

changed to its simplest form of expression, and will equal 
a sphere of the same denomination, of the value of their 
difference, combined with a cylinder of the same strength 
as the cylinder given, but of opposite sign and axis. For 
example: +4 S. O — 1 cyl. axis 180. The minus one 
cylinder is refracting in the 90 degree meridian, therefore it 
reduces the strength of the +4 S. in this axis, making it a 
plus 3. The horizontal or 180 degree meridian of the plus 
4 S. has not been altered, but still remains -f~4, and the re- 
sult is, plus 3 in the vertical meridian and plus 4 in the 
horizontal meridian, equaling, therefore, -f-3 S. ^ -j- 1 
cyl. axis 90. 

The following rule will be of service in making this 
change, and, in fact, this rule will apply in any instance 
where the sphere and cylinder are of different denomina- 
tion, no matter what their respective strengths may be : 

Rule. — Subtract the less from the greater *, and to the result 
prefix the sign of the greater ; combine with this the same 
strength cylinder, using the opposite sign and opposite axis. 
Example : +2.25 S. O — 0.75 cyl. axis 75 degrees ; sub- 
tracting the less from the greater ( — 0.75 from +2.25), and 
prefixing the sign of the greater ( + ), will leave -f 1.50 S. ; 
and combining with this the same strength cylinder (0.75), 
with opposite sign and axis (-j- and 165), will be +0.75 
cyl. axis 165. Result, +1.50 S. O +0.75 cyl. axis 165. 

Combining a Sphere and Cylinder of the Same De- 
nomination. — This is recognized as the minimum or 
simplest form of expression, and is seldom changed. For 
example : — 2 S. O — 6 cyl. axis 180 is considered as the 
thinnest lens and the one with the least weight that can be 
made by such a combination. It may be changed, how- 
ever, by the reverse of the rule above given, and will equal 
— 8 S. O -f 6 cyl. axis 90. 



52 REFRACTION AND HOW TO REFRACT. 

Combining Two Cylinders of Different Denominations 
with Opposite Axes. — Commonly called crossed cylinders. 
Such combinations can be written in three ways : 

1. -J-Cyl. 3 — cyl. axes opposite. 

2. -\- Sphere 3 — c yl. (cylinder stronger than sphere). 

3. — Sphere -f c yl- (cylinder stronger than sphere). 

For example : — 1.00 cyl. axis 180 3 ~\~ 2 -S° C Y^- ax ^ s 9° 
may be changed to one of the following : 

— 1. 00 S. O +3-5° cyl. axis 90; or — 
-f 2.50 S. 3 — 3.50 cyl. axis 180. 

The first formula shows that the vertical meridian must 
always be — 1 and the horizontal or 180 meridian must 
always be +2.50, and with this clearly in mind, the second 
and third formulas will be understood. In the second 
formula ( — 1.00 S. O +3- 50 cyl. axis 90) the +3.50 cyl. 
is only equal to +2.50, as it has 1 D. neutralized by — 1 
of the — 1 sphere. In the third formula (-J- 2.50 S. O 

— 3.50 cyl. axis 180) the — 3.50 cylinder is only equal to 

— 1. 00 cylinder, as it has — 2.50 neutralized by -J-2-50 of 
the sphere. 

In any spherocylindric combination the meridian in which 
the axis of the cylinder lies has the strength of one lens, and 
the meridian opposite to the axis of the cylinder has the com- 
bined values of sphere and cyli?ider — i. e. y — 1.00 S. O 
+ 3.50 cyl. axis 90 means - — 1.00 on the axis (90) of the 
cylinder, and opposite to the axis therefore at 180, it equals 
+ 2.50 (—1 and +3-50). 

Crossed cylinders in themselves are seldom ordered in a 
prescription, preference being given to a spherocylindric 
combination. When to order a plus sphere with a minus 
cylinder, and when to order a minus sphere with a plus 
cylinder, depends upon the individual lenses. For example : 



optics. 53 

-J-0. 50 cyl. axis 90 O — 5. 00 cyl. axis 180 equals -I-0.50 
S. O — 5.50 cyl. axis 180 or — 5 S. O -f- 5. 50 cyl. axis 90. 

Preference would be given to the plus sphere combina- 
tion, on account of thinness and lesser weight of the lens. 
The following formula, — 1 cyl. axis 1 80 degrees ^ -j- 3 
cyl. axis 90 degrees, equals — 1.00 S. ^ +4 cyl. axis 90, 
or +3 S. ;3 — 4 cyl. axis 1 80, and for similar reasons 
preference would be given to the minus sphere combination. 
Whichever combination makes the thinnest and lightest 
weight of glass is the one to be ordered, as a rule. 

The student should practise these combinations at the 
trial-case, and be able at a glance to change one formula 
into another without diagram or rule. 

Prescription Writing. — In writing prescriptions for 
lenses the right eye is indicated by one of three signs — R, 
Rt, or O. D., the latter from the Latin for right eye, 
Oculus Dexter. The left eye is also indicated in one of 
three ways — L, Lt, or O. S., the latter from the Latin 
for left eye, Oculus Sinister. 

A prescription may call for any one of the following : 

-(-Sphere, written - 4 D. or +4.00 D. S. or +4 S. or -[4 s ph« 

— Sphere, written — 2 D. or — 2.00 D. S. or — 2 S. or — 2 sph. 

— Cylinder, written —4.00 D. C. or -[-4 C. or -(-4 cyl. (axis as indicated). 

— Cylinder, written — 2.00 D. C. or — 2 C. or — 2 cyl. (axis as indicated). 

-•-Sphere and 4, cylinder, written -f 2.00 S. 3 -(-2- 00 cyl. axis 90 degrees. 

— Sphere and — cylinder, written — 2.00 S. 3 — 2.00 cyl. axis 180 degrees. 

-r Sphere and — cylinder (cylinder stronger than sphere), -f 2.00 S. 3 — 3.00 

cyl. axis 180 degrees. 
— Sphere and -(-cylinder (cylinder stronger than sphere), — 2.00 S. 3 -(-3.00 

cyl. axis 90 degrees. 

A plus cylinder and minus cylinder may be prescribed, 
and, if so, their axes must be at right angles to each other. 
An occasional exception to this may be found in irregular 
astigmatism. Or a prism with its base indicated may be 



54 REFRACTION AND HOW TO REFRACT. 

added in any one of the foregoing formulas ; for example : 
— 2 S. O — 2.00 cyl. axis 180C2A base in ; or the direc- 
tion of the base may be abbreviated as follows : B. L, 
meaning base in ; B. O., meaning base out ; B. U., meaning 
base up ; and B. D., meaning base down. 

Prescriptions are never written for two spheres. 

Prescriptions are never written for two cylinders at the 
same axis. 

Prescriptions are never written -for two cylinders at axes 
other than those at right angles to each other, except, as 
just noted, in irregular astigmatism. 

For obvious reasons prescriptions are never written for a 
sphere and two cylinders except in irregular astigmatism. 

Recognition of Lenses. 

A convex sphere is thick at the center and thin at the 
edge. It has the power of converging rays of light ; hence, 
if strong, it is a burning glass. Objects viewed through a 
convex lens as it is moved before the eye, from left to 
right and right to left or up and down, appear to move 
in an opposite direction to that in which the lens is 
moved. The weaker the lens, the slower the object 
appears to move ; and the stronger the lens, the faster the 
apparent movement of the object. A convex lens being a 
magnifier, has the effect of making objects appear larger 
and closer when it is moved away from the observer's eye ; 
or if brought toward the eye, objects already enlarged 
appear smaller and more distant. 

To Find the Optic Center of a Convex Lens. — Look- 
ing at a vertical straight line and passing a convex 
lens before the eye from left to right has the effect of 
displacing toward the right edge of the lens that portion of 
the line seen through the lens (see Fig. 54), and as the 
lens is slowly moved still further to the right, the displaced 



OPTICS. 



55 



portion of the line will finally coincide with the original 
straight line, making one continuous line through the lens. 
(See Fig. 55.) Marking this straight 
line on the surface of the lens, and 
then turning the lens to the opposite 
meridian and repeating the examina- 
tion, and marking the lens as before, 
the optic center will be in the lens 
beneath the point of intersection of the 
two lines. (See Fig. 56.) 

A concave sphere is thick at the 
edge and thin at the center, and has 
the power of causing rays of light 
to diverge. When moved before the 
eye from left to right and right to left 
or up and down, objects appear to move in the same direc- 
tion as that in which the lens is moved. 

A concave lens being a minifier, makes objects appear 




Fig. 54. 





Fig. 55. 



Fig. 56. 



smaller and more distant as the glass is moved away from 
the eye, and if brought closer to the eye, makes objects 
apparently small appear somewhat larger and nearer. 



56 



REFRACTION AND HOW TO REFRACT. 



Looking at a straight edge or line through a concave 
sphere, and passing the lens from left to right, the portion 
of the line seen through the lens appears displaced toward 
the center of the lens (see Fig. 57), and as the lens is still 
further moved to the right, the displaced portion of the 
line finally coincides with the original straight edge, as in 
figure 55. 

The optic center of a concave lens is found in the same 
way as the center of a convex lens. 

A Convex Cylinder. — When a convex cylinder is moved 





Fig. 57. 



Fig. 58. 



in front of the eye in the direction of its axis, objects looked 
at do not change their positions ; but when the lens is 
moved in the direction opposite to its axis, the movement 
of the object is the same as that of a convex sphere. Look- 
ing at a straight edge through a convex cylinder, and 
rotating it, has the effect of displacing away from its axis 
that portion of the straight edge seen through the lens. 
(See Fig. 58.) 

A Concave Cylinder. — When a concave cylinder is 
moved in front of the eye in the direction of its axis, ob- 



OPTICS. 



57 



jects looked at do not change their positions ; but when the 
lens is moved in the direction opposite to its axis, the 
movement of the object is the same as that of a concave 
sphere. Looking at a straight line through a concave 
cylinder, and rotating it, has the effect of displacing toward 
its axis that portion of the straight line seen through the 
lens. (See Fig. 59.) A circle viewed through a strong con- 
cave cylinder appears as an oval with its long diameter cor- 
responding to its axis. (See Fig. 60.) A circle viewed 
through a strong convex cylinder appears as an oval with 
its long diameter opposite to its axis. In place of using a 





Fig. 59. 



Fig. 60. 



straight line or straight edge to find the optic center of a 
sphere or axis of a cylinder, two lines at right angles may 
be substituted (see Fig. 56) or a protractor may be used. 

A Prism. — Objects viewed through a prism are dis- 
placed toward its apex, and that portion of a straight line 
seen through the prism never coincides with the straight 
line. 

Neutralization of Lenses. — Having determined from 
the foregoing description what the character of an indi- 
vidual lens may be, then to neutralize its effect or find out 
its strength a lens of opposite character is taken from the 



58 REFRACTION AND HOW TO REFRACT. 

trial-case and held in apposition to it, and the two lenses 
are moved in front of the eye as a distant object is 
observed. That lens or combination of lenses which stops 
all apparent movement of the object is the correct neu- 
tralizing lens. Spherocylindric lenses are neutralized by 
finding out what sphere will correct one meridian and what 
sphere will correct or neutralize the opposite meridian ; for 
example, if a minus 2 S. stops all movement in one 
meridian and minus 3 S. stops all movement in the other 
meridian, then the lens being neutralized will be plus 2 S. 
combined with a plus 1 cylinder. Or after a sphere neu- 
tralizes one meridian, a cylinder may be combined until the 
other meridian is neutralized. 



CHAPTER II. 

THE EYE.— THE STANDARD EYE. — THE CARDINAL 
POINTS.— VISUAL ANGLE.— MINIMUM VISUAL AN- 
GLE.— STANDARD ACUTENESS OF VISION.— SIZE OF 
RETINAL IMAGE.— ACCOMMODATION.— MECHANISM 
OF ACCOMMODATION.— FAR AND NEAR POINTS.— 
DETERMINATION OF DISTANT VISION AND NEAR 
POINT.— AMPLITUDE OF ACCOMMODATION. — CON- 
VERGENCE.— ANGLE GAMMA.— ANGLE ALPHA. 

The Eye. — While the eye is considered as the organ of 
vision, yet its function is to form upon its retina an inverted 
image of any object looked at ; and if the retinal image is 
distinct, the object will appear distinct ; if the retinal image 
is blurred, the object will appear blurred. By means of 
the optic nerve and tract the retinal impression or image 
is placed in communication with the brain, which interprets 
the image and completes the visual act. 

The Standard Eye. — For purposes of exact calcula- 
tions it has been found necessary to project a standard or 
schematic eye, whose nodal point (optic center) shall be 
seven millimeters back of the anterior surface of the cor- 
nea and fifteen millimeters from the fovea (Helmholtz). 
Allowing one millimeter for the thickness of the choroid 
and sclera, such an eye would have an anteroposterior 
measurement of about twenty-three millimeters. Parallel 
rays of light passing into such an eye in a state of rest 
would focus on the macula. 

Cardinal Points (Fig. 61). — Images formed upon the 
retina are the result of refraction by three refracting sur- 
faces and three refracting media. The refracting surfaces 

59 



60 REFRACTION AND HOW TO REFRACT. 

are the anterior surface of the cornea and the anterior and 
posterior surfaces of the crystalline lens. The refracting 
media are the cornea (and aqueous humor forming a convex 
lens), the crystalline lens, and the vitreous humor. These 
refracting surfaces and media represent a compound dioptric 
system, centered upon the optic or principal axis — i. e., a 
line drawn from the pole of the cornea to a point between 
the nerve and fovea. 

On the principal axis are situated the anterior and poste- 
rior principal foci (see p. 33), the anterior and posterior nodal 




Fig. 61. 

points, and the anterior and posterior principal points. The 
anterior principal focus is situated upon the optic axis 
I 3-745+ mm - m front of the corneal apex. The pos- 
terior principal focus is situated 15.61 -}- mm. back of the 
posterior surface of the lens. The nodal points are situ- 
ated about 7 mm. back of the cornea, and correspond 
approximately to the optic center of this compound re- 
fracting system ; and as they are so close together, they are 
considered as one for all purposes in the study of the for- 
mation of images. The first or anterior principal point is 



MINIMUM VISUAL ANGLE. 6 1 

situated 1.75 mm. back of the anterior corneal surface, and 
the second or posterior principal point is situated 2.10 mm. 
behind the anterior surface of the cornea. The principal 
points are so closely situated that they are considered as 
one. The anterior focal distance equals 15.49-)- mm. and 
the posterior focal distance equals 20.71+ mm. 

The Visual Angle, or Angle of View. — The visual angle 
is the angle formed by rays of light from the extremes of an 
object passing to the nodal point of the eye ; or the visual 
angle may be denned as the angle which the object subtends 
at the nodal point of the compound refracting system of 
the eye. Rays of light from the extremes of an object 




Fig. 62. 

directed to the nodal point of the eye pass through unre- 
fracted, and continuing their straight course, fall upon the 
retina, forming an inverted image of the object. (See Fig. 
62.) 

The size of the retinal image depends upon the size and 
the distance of the object from the nodal point of the eye. 
Objects, therefore, which are seen under the same visual 
angle must have the same sized retinal image. (See Fig. 

63.) 

If the arrows 1, 2, 3, and 4 represented a child, a man, a 
tree, and a church, respectively (some distance apart), they 
would form the same sized retinal images, and if the eye 
were guided alone by the size of the retinal image, it would 



62 



REFRACTION AND HOW TO REFRACT. 



judge erroneously; but, by experience, distance and com- 
parison of size are brought into judgment. 

If, however, arrows 2, 3, and 4 are placed at the side of 
arrow 1, then their resulting images would increase in size 
according to the size of their respective visual angles. (See 
Fig. 64.) 

The nearer an object to the eye, the larger the visual 




Fig. 63. 

angle and retinal image ; the further away an object from the 
eye, the smaller the visual angle and retinal image. An ob- 
ject, to retain the same sized visual angle, must, therefore, be 
made larger the further it is removed from the eye ; this is 
demonstrated in figure 63, where arrow 1, to be seen 




Fig. 64. 



under the same visual angle which it has at present, would 
have to be as large as arrow 4, at the distance of arrow 4. 
Minimum Visual Angle.— This is the smallest visual 
angle in which a standard eye can still recognize an object 
and give it a name ; this angle is also spoken of as the 



SIZE OF RETINAL IMAGES. 63 

limiting angle of vision. In figure 65, for example, the 
letter D at a distance of six meters is recognized as the 
letter D : it is plainly seen ; but if placed beyond six meters, 
it would form a smaller visual angle, and could not with 
certainty be called D. 

To be seen at a distance of twelve meters and still 
occupy this same visual angle, D would have to be made 
twice as large — i. i\, the size of F ; and to be seen at 
twenty-four meters, it would have to be four times its pres- 
ent size, or the size of P. Thus, while the letter D, seen 
clearly at six meters, would have to be made proportion- 
ately larger as it is removed from the eye, then to occupy 




Fig. 65. 

the same visual angle it would have to be made smaller 
if brought closer to the eye and kept within this limiting 
angle. In figure 65 D, F, and P can be seen closer to 
the eye than their respective distances call for ; but the pur- 
pose is to find the greatest distance from the eye at which 
they can be seen, as this represents the maximum acuteness 
of vision, or maximum sharpness of sight. 

Standard Acuteness of Vision. — As it was necessary for 
purposes of calculation to have a standard or emmetropic 
eye, so it is essential to have a standard acuteness of vision 
which will be consistent with the standard or emmetropic 
eye, and thus have some method of recording numerically 
any departure from this standard visual condition. 



64 REFRACTION AND HOW TO REFRACI. 

The standard acuteness of vision is the power of the eye to 
distinguish letters and characters occupying an angle of five 
minutes. Every letter is, therefore, so proportioned that it 
will measure just five minutes in the vertical and hori- 
zontal meridians, and be reducible to twenty-five parts or 

squares, each measuring 



m 



one minute vertically and 

horizontally.* (See Fig. 

66.) 

■~~ „ ' ' C J ' Figure 67 shows the 

Fig. 66. Fig. 67. fa ' 

letter F drawn in a five- 
minute square, and each stroke of the letter, and space 
between the strokes, measuring just one minute in width. As 
the tangent of half the angle of five minutes is expressed 
by the decimal .001454, then to calculate the size of 
any letter or character which should be seen clearly and dis- 
tinctly by the standard eye at a certain definite distance, it 
is necessary to multiply the distance in millimeters by this 
tangent of the angle of five minutes. Letters or characters 
made on this scale are called standard letters. For ex- 
ample, letters to be seen under an angle of five minutes at 
a distance of one meter (1000 mm.) would have to be 
1.454 mm. square (1000 X .001454). At six meters 
(6000 X .001454) = 8.7 mm., etc. 

Size of Retinal Images. — The size of the retinal image 
depends upon two factors — the size of the object itself and 
its distance from the nodal point. In the standard eye it 
has been stated that the nodal point was 7 mm. back of 
the cornea and 1 5 mm. in front of the retina ; then an 
object 8.7 mm. square situated 6000 mm. in front of the 

* There are two letters in the alphabet which are exceptions to this rule, 
L and 0. L can be seen under an angle of two minutes and O can be seen 
under an angle of three minutes. 



SIZE OF RETINAL IMAGES. 6$ 

eye would have a retinal image 6 q jjo °^ ^-7> or °-° 2 ~\~ mm., 
and this is the size of the retinal image in a standard eye, 
looking at a standard letter at six meters' distance. A good 
rule for finding the size of the retinal image is to multiply 
the height of the object by the nodal distance and divide 
by the distance. In other words, the size of the retinal 
image is to the size of the object as their respective dis- 
tances from the nodal point. 

Refraction in ophthalmology has most to do with eyes 
whose measurements are not according to the standard or 
emmetropic condition, and which have their retinas closer 
to or further from the nodal point than 1 5 mm. (spoken of 
as ametropic). The 

retinal images in ^^r M 

such eyes will be 
smaller in the former 
and larger in the 
latter. (See Fig. 68.) 

Accommodation. Fig. 68. 

— This may be de- 
scribed as the power of the eye to focus rays of light upon its 
retina from different distances at different times. In other 
words, the eye can not focus rays of light upon its retina from 
different points at one and the same time. For example, the 
point of a pencil held six inches in front of the eye is not seen 
clearly (is hazy) as the eye looks at a printed page thirteen 
inches beyond ; and, vice versa, the printed page is not seen 
distinctly if the point of the pencil is looked at. In the 
study of convex lenses it was noticed that when an object 
was brought closer than infinity, the focus of the lens was 
correspondingly lengthened ; and so, in the photographer's 
camera, to keep the focus on the ground-glass or sensitive 
plate as the object is brought toward the camera, it is 




66 REFRACTION AND HOW TO REFRACT. 

necessary to push the lens forward by means of the accor- 
dion plaits ; but the human eye does not lengthen or 
shorten in this way. Normally, the eyeball is inextensible, 
and to accomplish this same purpose the ciliary muscle 
must contract, causing the crystalline lens to become more 
convex, and thus keep the rays of light entering the eye at 
a focus upon the fovea. 

The Mechanism of Accommodation. — To appreciate 
this, it is necessary to understand something of the anatomy 
of the ciliary body, of which the ciliary muscle is a part 
The ciliary body is circular in form and occupies a small 
(3 mm.) area in the eye, just beneath the sclera, at its cor- 
neal junction. (See Fig. 61.) In section the ciliary body is 
triangular in shape, the base of the triangle measuring about 
0.8 mm. and facing toward the anterior chamber, the apex 
of the triangle extending backward beneath the sclerotic. 
The ciliary body lies in apposition to the sclera, but has 
only a very minute attachment to it, at the sclerocorneal 
junction, called the ligamentum annulare, or pectinatum. 
That portion of the ciliary body lying next to the hyaloid 
membrane of the vitreous humor is composed of folds, 
known as the ciliary processes, seventy or more in number. 

A portion of the ciliary body is composed of muscular 
fibers disposed in flat bundles, which interlace with each 
other, forming a sort of plexus, and called the ciliary mus- 
cle. This muscle, by the character of its fibers, has been 
subdivided into three parts : (1) Meridional ; (2) radiating ; 
and (3) circular or sphincter fibers. The meridional are 
the longest, lie next to the sclerotic in lamellae, parallel 
with it, and pass back to join the choroid coat of the eye, 
forming what is known as the tensor choroideae, or muscle 
of Brticke or Bowman. The radiating fibers are fan-shaped, 
few in number, and scattered through the ciliary body. 



THE MECHANISM OF ACCOMMODATION. 6j 

The circular or sphincter fibers — also called annular — are 
sometimes referred to as the muscle of Muller, or com- 
pressor lentis, and are the most important fibers in the 
consideration of accommodation ; they form a sphincter 
ring concentric with the equator of the lens. Attached to 
the ciliary body, well forward on its inner side, near the 
base of the triangle, is the ligament of the lens (zonule of 
Zinn), and it in turn sends fibers to the anterior and poste- 
rior capsule of the lens. This ligament of the lens occu- 
pies an interval of about 0.5 mm. between the ciliary body 
and the periphery of the lens, and is a constant factor in 
all conditions of the healthy eye. 

During the act of accommodation the following changes 
take place in the eye : 

1. The ciliary muscle contracts. 

2. The ciliary muscle (sphincter), by contracting, makes 
a smaller circle. 

3. The tensor choroideae draws slightly upon the choroid 
(compressing somewhat the vitreous body), and these two 
sets of fibers, sphincter and meridional, acting together, 
relax the ligament of the lens, with the result that — 

4. The lens fibers, no longer held in check, become re- 
laxed, and by their own inherent quality (elasticity) allow 
the lens to become more convex, especially on its anterior 
surface. 

5. The anterior surface of the lens being made more 
convex, approaches the cornea. 

6. The posterior surface of the lens becomes slightly 
more convex, but retains its position at the pole. 

7. The lens axis is lengthened, but the equatorial diame- 
ter diminishes, thus keeping up the uniform interval between 
the equator of the lens and the ciliary body, as previously 
referred to. The lens docs not increase in volume. 




68 REFRACTION AND HOW TO REFRACT. 

8. The anterior chamber becomes slightly shallower at 
the center and deeper in the periphery. 

9. That portion of the iris resting upon the anterior cap- 
sule of the lens is pushed forward, espe- 
cially at its pupillary edge. 

10. The iris contracts, producing a 
smaller pupil ; but it must be remembered 
that contraction of the iris is not an essen- 
tial condition in accommodation. The shape 
of the cornea is not changed during con- 
traction of the ciliary muscle. 

The following table shows the compara- 
tive measurements of a lens at rest and 
Fig. 69. 

during the height of accommodation in a 
healthy emmetropic eye of ten years. The dotted lines 
in figure 69 indicate the changes in the shape of the lens 
at the height of accommodation. 

Height of 

At Rest. Accommodation. 

Radius of curvature of anterior surface of lens, io mm. 6 mm. 

" " posterior " " . 6 " 5.5 " 
•Distance from anterior surface of cornea to ante- 
rior surface of lens, _ . . . 3.6 " 3.2 " 

Anteroposterior diameter, on axis, 3.6 " 4 " 

Distance from anterior surface of cornea to poste- 
rior surface of lens, 7.2 " 7.2 " 

Equatorial diameter, 8.7 " 8.2 " 

Far Point. — Latin, punctum remotum ; abbreviated p. r. 
or r. The far point may be defined as the greatest distance 
at which an eye has maximum sharpness of sight, or the 
most remote point at which the eye, in a state of rest, has 
maximum acuity of vision. Infinity (sign of infinity, oo ) 
is the far point of an emmetropic eye. 

The standard or emmetropic eye, when looking at distant 
objects, receives parallel rays of light at a focus upon its 



AMPLITUDE OF ACCOMMODATION. 69 

fovea (Fig. 70), and also emits parallel rays ; under these 
conditions the ciliary muscle is not acting, the eye is in a 
condition of complete repose, of rest, of minimum refrac- 
tion, and is adapted for its far point. 

Near Point. — Latin, pwictum proximum ; abbreviated 
p. p. or p. This may be defined as the nearest point at 
which an eye has maximum sharpness of sight, or the 
nearest point to the eye at which it has distinct vision, the 
lens is in the condition of greatest convexity, of maximum 
refraction. 

Amplitude of Accommodation. — This is also called the 
range * or power f of accommodation, and may be de- 
fined as the difference between the refraction of the eye in 
a state of rest (or adapted for its far point) and in a condi- 
tion of maximum refraction, or adapted for its near point. 
For example, an emmetropic eye has infinity for its far 
point, and if 10 cm. distance is its near point, then the dif- 
ference between the lens adapted for infinity and 10 cm. will 
be 10 D., as 10 cm. represents the focal length of 10 D. In 
other words, there is no accom- 
modation used for infinity, but 
there is an accommodation of 
10 D. for the near point, which 
is the amplitude or power of 
accommodation. The emme- 
tropic eye in a state of accom- Fig. 70. 
modation adds on to the ante- 
rior surface of its lens what is equivalent to a convex 
meniscus. Figure 70 shows an emmetropic eye at rest 
receiving parallel rays of light at a focus upon its retina, 

* Range applies to the space between the far and near points. 
| Power applies to the force or strength or diopters necessary to change 
the refraction from the far to the near point. 




JO REFRACTION AND HOW TO REFRACT. 

and it also shows the same eye in its maximum state of 
accommodation for a point 10 cm. distant; the broken 
line representing what is equivalent to a convex meniscus, 
added to the anterior surface of its lens. 

When the distance of the near point is known in inches 
or centimeters, the equivalent in diopters is found by divid- 
ing 40 by the near point in inches, or by dividing 100 by 
the near point in centimeters. The near point being 10 cm., 
or 4 inches (10 into 100 or 4 into 40) the amount of accom- 
modation will be 10 D. 

In the study of healthy emmetropic eyes it has been 
found that the power of accommodation gradually dimin- 
ishes as the eye passes from youth to old age. This is 
the result of one or more changes : the lens fibers lose 
their elasticity, becoming sclerosed, or the ciliary muscle 
grows weak, or both of these changes may exist together. 
Rarely the cornea may flatten. A knowledge of the power 
of accommodation is absolutely essential, so that any vari- 
ations from the standard condition may be noted. The fol- 
lowing table gives the ages from ten to seventy-five years, 
respectively, with five-year intervals, and the near point 
consistent with each, as also the amplitude of accommoda- 
tion for each period. 







A 


MPLITUDE 






Amplitude 


EAR. 


Near Point. 


IN 


Diopters. 


Year. 


Near Point. 


in Diopters. 


IO 


7 cm. 




H 


45 


28 cm. 


3.5 


15 


8.5 « 




12 


5o 


40 " 


2-5 


20 


10 " 




IO 


55 


55 " 


i-75 


25 


12 " 




8-5 


60 


100 " 


1 


30 


14 " 




7 


65 


U2> " 


o-75 


35 


18 << 




5-5 


70 


400 " 


0.25 


40 


22 " 




4-5 


75 


00 





This table of near points applies only to emmetropic eyes 
or those eyes which are made emmetropic by the adjust- 



FAR POINT. /I 

ment of suitable correcting lenses. The table of amplitudes, 
however, is the same, with a few exceptions, for all eyes of 
whatever degree or amount of ametropia. 

For a better appreciation of the amplitude of accom- 
modation it is necessary to understand the two forms of 
eves already referred to in figure 68. 

First, the eye which has its retina closer to its refractive 
media than the principal focus ; such an eye is spoken of 
as a short or hyperopic eye. (H in Fig. 68.) (Hyperopia : 
Greek, b-ep, over ; and a>4', eve -) 

This eye in a state of rest (under the influence of atro- 
pin) will emit divergent rays of light, and is, therefore, in a 




Fig. 71. 



condition to receive only convergent rays of light at focus 
upon its retina. (See Fig. 71.) Parallel rays would not 
focus upon the retina of such an eye, but, if possible, would 
focus back of the retina. 

Second, the eye that has its retina beyond the principal 
focus of its dioptric media (M in Fig. 68) ; such an eye is 
spoken of as a long or myopic eye (Greek, fiuetv, to close ; &<p, 
eye). This eye always emits convergent rays, and is, there- 
fore, in a state to receive divergent rays of light at a focus 
upon its retina. (See Fig. 72.) Parallel rays would not 
focus upon the retina of a myopic eye, but in the vitreous 
in front of the retina. 

The Far Point of a Hyperopic Eye. — This must neces- 



72 REFRACTION AND HOW TO REFRACT. 

sarily be negative (see Fig.. 71), and is found by projecting 
the divergent emergent rays backward to the imaginary 
point behind the retina from which they appear to have 
diverged. A hyperopic eye, to receive parallel rays of 
light at a focus upon its retina, must, therefore, accommo- 
date, and the amount of accommodation thus exerted will 
remove the near point just that much from the eye as com- 
pared with an emmetropic eye. For example, according to 
the table of amplitudes just given, an eye at twenty years has 
10 D. of accommodation, but if it uses 2 D. of this to make 
rays of light parallel, then it only has 8 D. left to accom- 
modate inside of infinity, with the result that the near point 
comes to only (8 into 100) 12.5 cm. ; or an eye which is 




Fig. 72. 

twenty-five years old has an amplitude ofaccommodation of 
8.5 D., and if it has to use 4.5 D. for infinity, it would have 
(4 into 100) a near point of 25 cm. (10 inches). 

The Near Point of a Hyperopic Eye. — From the de- 
scription just given it will be seen at once that the near 
point in hyperopic eyes is always further removed than in 
the emmetropic eye for a corresponding age, and that the 
near point depends upon the amount of accommodation 
that is left after the eye has accommodated for infinity. 

The Far Point of a Myopic Eye. — This is always posi- 
tive and situated some place inside of infinity. It is found 
by uniting the convergent emergent rays. (See Fig. 72.) 



FAR POINT. 



73 



The far point of a myopic eye is the result of its strong 
refracting power or the distance of its retina beyond the 
principal focus of its dioptric media. The retina and far 
point of a myopic eye are conjugate foci. (See Fig. 72.) 
The myopic far point is equivalent to just that much refrac- 
tion in excess of the emmetropic eye. An emmetropic eye 
under the influence of atropin would require a -f 2 S. 
placed in front of it to make rays of light focus upon its 
retina from a distance of 50 cm., and rays of light from the 
retina of this eye with a +2 S. in front of it would focus 
at 50 cm. This eye, then, equals a myopic eye of 2 D. 
This myopic eye would have a far point of 50 cm. Where 
the rays of light meet as they come from a myopic eye in a 
state of rest is its far point. 

The Near Point of a Myopic Eye. — This is always 
closer than in the emmetropic eye for a corresponding age, 
and depends upon the distance of its far point. For exam- 
ple, an eye at twenty-five years has 8.5 D. amplitude of 
accommodation, but if it has a far point of 70 cm., then its 
near point will be represented by 8.5 D. and 70 cm., — i. e. y 
1.5 D., which would equal 10 D., or a near point of 10 cm. 
The following table gives the comparative near points in an 
emmetropic eye, a hyperopic eye of 2 D., and a myopic eye 
of 2 D. : 



Age. 


10 

7 


15 

8-3 


20 


25 
12 


30 


35 


40 


45 

2S 


50 

40 


55 
55 


60 

IOO 


65 
133 


70 

400 


Emmetropia, p.p. 


10 


18 


22 


2 D. Hyperopia, " 


8.3 


10 


125 


ib 


20 


28.5 


40 


66 


200 


00 


— 


— 


— 


2 D. Myopia, 


6 


7 


8-3 


10 


11 


13 


15-3 


18 


22 


25 


33 


36 


44 



75 



Determining the Vision. — This may be considered as 
the method of finding out what an eye can see without any 
lenses placed in front of it ; in other words, determining the 
vision may be defined as ascertaining the seeing quality of the 

7 



74 REFRACTION AND HOW TO REFRACT. 

unrefracted eye. The refraction of an eye should never be 
confounded with the visual quality, as refraction applies to 
the refractive media ; for example, an emmetropic eye with 
a hemorrhage at the fovea would be practically without 
visual quality, and yet its refraction or refractive condition 
would be standard. The most acute vision is at the fovea 
and the region immediately surrounding it, but this sensi- 
bility diminishes as the fovea is departed from and the per- 
ipheral portion of the retina approached ; this is due to the 

fact that the cones are as 
close as 0.002 mm. at the 
macula, and not so close or 
numerous in the forepart of 
the eye-ground. 

Test-type or Test-let- 
ters for Distant Vision. — 
To determine the vision we 
employ cards on which are 
engraved test-type or letters 
of various sizes, constructed 
"l-?:."."." "Z:Z~" so that each letter subtends 

Fig. 73.— Randall's Test-letters. Block an angle of five minutes, as 
letters in black on cream-colored on 

cards. suggested by Snellen, and 

described on page 64. 
Figure 73 shows such cards of test-letters, reduced in size. 
The Roman characters just over the top of the letters indi- 
cate the distance in meters that the letters should be seen by 
the standard eye, and the little figures at the left of the letters 
indicate the equivalent distance in English feet. The top 
letter should be seen at 60 meters, and the bottom letters 
at 3 meters ; the intervening letters are to be seen at the 
respective distances indicated. As it is not unusual to find 
eyes that have a seeing quality better than that obtained 



E 

C P 


c 

C P 


L 6 A 


va 

O A L 


T G Y D 


G Y D T 


F A V H U 


V F H U A 


D L O E C N 
C P A R"C E O L, 
athye'vkufd 

OPOHCLBEBVH 


L E C*N D O 
GEO R°l* C P A 

v k u r x>'a t h t b 

D«»»»oro«eL 



TEST-TYPE OR TEST-LETTERS FOR DISTANT VISION. 75 



with Snellen's type constructed on the angle of five min- 
utes, Dr. James Wallace has constructed letters which sub- 
tend an angle of only four minutes. Such a card is shown 
in figure 74 and has a large field of usefulness. While 
test-cards are usually white or cream-colored, with black 
letters, Gould has white letters constructed on black 
cards. (See Fig. 77.) As white stimu- 
. C lates the retina and black does not, it 

* I ' will be recognized at once that in one 

instance the card, and in the other the 
letters, produce the retinal stimulation. 
The white letters seem to stand out from 
the black card al- 
most as if they were 
embossed, giving a 
clear-cut edge and 
most soothing effect 
to the eye under ex- 
amination, and can 
be recognized when 
subtending a much 
smaller angle than 
the black letters. To 
avoid reflection, this card should be 
hung at an angle. 

For aliens who do not know the 
English letters, and for illiterates, a 
special card has been made, known as 
the illiterate or "dummy" card, with 
characters consisting of lines shaped 

like the capital letter " E," and made to conform to the five- 
minute angle. As these letters are variously placed, the 
patient is asked to tell, or indicate with his finger or fingers, 



-OOO 
-X X Y V 

■9\ 8 8 H M 
-1 «* O O A H 
■ V V T 3 s x a 
••AooeajT 



Fig. 74. — Four Min- 
ute Letters of Dr. 
J. Wallace. This 
card is constructed 
principally for re- 
flection purposes. 
(See Fig. 7.) 



LU 

E U 3 
e 3 in m 

U3EHB 
ill m e u 3 e 



Fig. 75- 



7 6 



REFRACTION AND HOW TO REFRACT. 




the direction in which the prongs of the "E" point: up, 
down, to the right or left. This illiterate card (see Fig. 
75) is much to be preferred to the German, Hebrew, and 
" figure " cards occasionally displayed in clinics. 

Kindergarten Card. — To keep the attention of children 
who do not know the alphabet, or who attend a Kinder- 
garten school, the author has 
prepared a card of test objects 
as shown in figure 76, These 
objects have been drawn up 
to the angle of five minutes, 
j «. though some of their com- 

^^^^ C l jf ponent parts do not measure 

§ DEI U P to one mmu te. 

Selection of Test-cards. 
— The surgeon should have 
several of these in duplicate 
with the order of the letters 
changed (Figs. 73, 77), as pa- 
tients not infrequently and 
unintentionally commit them 
to memory. Care should be 
exercised in the selection of 
test-cards, to see that each 
letter on the card measures 
up to the standard square of 
five minutes, as many of the 
A's and R's and N's, etc., on 
the old cards as seen in the 
shops measure six and seven minutes horizontally. It is 
a matter of choice with the surgeon whether to use test- 
cards with the block or Gothic letters. It is well to have 
both. 



o & u 

385 *"'• A 
rTi o n * * 

k. t h"b a • 

ODlHtKeD 
-rBORa*nft 

• niotooa 



Fig. 76. — Author's Test Objects or 
Kindergarten Card for Children 
and Illiterates. 



THE RECORD OF THE VISUAL ACUITY. 



77 



Method of Procedure. — The test-card should be hung 
on the wall with its ^| line five or six inches below the 
level of the patient's eyes, and illuminated by means of 
reflected artificial light. This is always a certain quantity, 
whereas daylight is too variable and not to be depended 
upon. The patient should be placed with his back toward 
any bright light, and at a distance of six meters from the card. 




Fig. 77. — Gould's Test-letters. Gothic letters in white on black cards. 



Sometimes the surgeon's office is not six meters long, and 
this distance must be obtained by using diagonal corners 
of the room or by using a plane plate-glass mirror and a 
specially prepared test-card with reversed letters (see Fig. 
74), the card being hung as many meters in front of the 
mirror as will make six meters when added to the length 



yS REFRACTION AND HOW TO REFRACT. 

of the office. While a distance of six meters is always to 
be preferred, yet if this can not be obtained, the surgeon 
may use a distance of four meters, but never less than this. 
Each eye should be tested separately, the fellow-eye being 
shielded or covered by a card or opaque disc held in front 
of it or placed in the trial -frame. The eye should never be 
held shut, and any pressure upon the eyeball must be 
avoided. 

The record of the visual acuity is usually made in the 
form of fractions, using Arabic or Roman notation ; 
figures usually indicate feet, and Roman letters usually sig- 
nify meters, though there is no fixed rule for this. How- 
ever expressed, the denominator indicates the size of the 
type which the eye reads, at the distance indicated by the 
numerator. For example, if at VI meters the eye reads 
the line of letters marked VI, then the record would be 
^j. This would be |-§- if the numerator and denomina- 
tor were expressed in feet. If the eye, at a distance of 
VI meters, reads only the letters on the XII line, then 
the record would be ^J-, or f£ (feet). If the top letter 
was the only one recognized at the distance of six meters, 
then the record would be j^- (meters), or -f^ (feet). If 
the eye reads the VI line, miscalling two letters, then the 
record could be made in one of three ways, each indicating 
the same thing. ^ ? ? (one question mark for each 
miscalled letter), or "^{ partly," would indicate that 
the eye saw ^, but not each letter correctly. This way of 
making the record is not so explicit as that with question 
marks. Or, — (- would mean that the eye saw all of 

rrrr— and some of the letters of 4rr ; but this, too, is not so 

VIIss VI ' ' ' 

definite as the first record and the one recommended. 



DETERMINING THE NEAR POINT. 79 

If the eye can not recognize any letter on the card at the 
distance of VI meters, then the card should be brought 
toward the patient, or the patient told to approach the card, 
until the eye can just make out the top letter and no more. 
If this is seen at IV meters, then the record will be — ; if at 
one meter, the record would be -^, etc. While it has 
been stated that the visual record is usually made in the 
form of common fractions, as just described, yet there are 
some who prefer to make the record in the form of deci- 
mals ; namelv, a vision of -— would be i.o, a vision of — — 
would be 0.50, or a vision of ^ryr r would be 0.25, or a 
vision of ^L would be o. 1. Most authorities prefer to make 
their records in the form of common fractions. 

In some instances the eye may not be able to distinguish 
any letter on the card, no matter how close it may be 
brought to the eye, and in such a case the vision is tested 
by holding the outstretched fingers between the patient's 
eye and a bright light (an open window), and a note is 
made of the greatest distance at which the eye can count 
fingers ; if at ten inches, the record would be " fingers 
counted at ten inches," or whatever the distance may be. 
This ability to recognize form is spoken of as " qualitative 
light perception." Eyes that are not able to recognize 
form may still be able to distinguish light from darkness, 
and this ability is tested by alternately covering and uncov- 
ering the eye as it faces a light, or as light is reflected into 
it from a mirror. If " qualitative light perception " is pres- 
ent, the vision is recorded L. P., which means " light per- 
ception," or the record may be made L. & S., which means 
practically the same thing, " light and shade." 

Determining the Near Point. — Having obtained and 
recorded the distant vision of an unrefracted eye, it is well 



80 REFRACTION AND HOW TO REFRACT. 

to also find out and note what is the nearest point to the 
eye at which small type can be made out ; this is spoken 
of as determining the near point. 

Test-type or Test-letters for Near Vision. — To deter- 
mine the near point, we employ cards on which are printed 
or engraved words or sentences, or a series of letters, so 
that each letter in each word or sentence shall subtend an 
angle of five minutes, at a given distance from the standard 
eye ; for instance, letters that are to be seen at one meter 
and occupy the angle of five minutes, must be 1.425 mm. 
square ; letters that are to be seen at half a meter distance 
must be 0.712 mm. square, etc. Most of the "near" cards 
in the market are very defective in this respect, and the near 
types of Jaeger are becoming obsolete, as they are not 
standard letters, but merely represent the various fonts of 
printers' type. The writer's card is one of Gothic type, as 
shown in figure 78. Another card in block letters is shown 
in figure 79. Above each series of letters is marked the 
greatest distance (D) at which the respective letters can be 
seen; these distances vary from 0.25 to 2 meters (25 to 
200 cm.), which are ample for all purposes in estimating 
the near point. 

Method of Procedure to Find the Near Point. — The 
patient is seated so that the light entering the room will 
come over his shoulder and fall upon the card of test-type 
held in front of him. The surgeon, to one side of the 
patient, holds the card in one hand and a meter stick in the 
other, the eye which is not being tested is covered with a 
card, and the patient is told to select the smallest type on 
the card which he can read or spell, and as he continues to 
do so (aloud), the surgeon gradually approaches the card 
to the eye until the patient says that the letters commence 
to grow " hazy " and he can scarcely decipher them; or 



TEST-TYPE FOR NEAR VISION. 



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82 



REFRACTION AND HOW TO REFRACT. 



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CONVERGENCE. 83 

another way is to hold the card close to the patient's eye 
and gradually withdraw it until he can just recognize the 
letters ; when this point is reached, the distance from the eye 
to the card is measured with the meter stick, and this dis- 
tance, as also the size of the type which was read, is care- 
fully recorded. For example, the patient selecting the type 
marked 0.50 D. and is able to read it as close as 8 cm. and 
no closer, the record will be "near point equals type O.50 
D. at 8 cm."; or abbreviated, would be "type 0.50 D. 
= 8 cm." 

In some instances the patient may not be able to read 
any of the near type without the aid of a glass, and if so, 
it will be necessary to place a plus sphere in front of the 
eye to assist in finding the near point ; for example, if a 

2 S. was employed, then the record might be "near 
point equals type O.50 D. at 12 cm. with +2 S.," or " -f-2 
S. = type 0.50 D. at 12 cm." 

Convergence. — Con, "together," and vergere, "to 
turn " ; literally, turning together. This is the power of the 
internal recti muscles (especially) to turn the eyes toward 
the median line ; to " fix " an object closer than infinity. 
Standard eyes, when looking at an object at a distance of 
six meters or more, are not supposed to converge ; the 
visual lines are spoken of as parallel and the power of con- 
vergence is in a state of repose. The angle which the 
visual line makes in turning from infinity ( 00) to a near 
point is called the angle of convergence, and the angle 
which is formed at One meter distance by the visual axis 
with the median line is called the meter angle, or the unit 
of the angle of convergence. (See 1, in Fig. 80.) 

If the visual line meets the median plane at y 2 of a 
meter, it has then two-meter angles of convergence ; at 
^ of a meter, four-meter angles of convergence, etc. Or 



8 4 



REFRACTION AND HOW TO REFRACT. 



five-meter angles means that the eye is converging to a 
point -|- of a meter distant. 

The size of the meter angle varies ; it is not the same in 
all individuals ; in fact, the meter angle is smaller in children 
than in adults, as a rule, on account of the shorter inter- 
ocular distance. In children this distance is about 50 mm., 
whereas in adults it is, on the average, 60 or 64 mm. 

While standard eyes, to see a point one meter distant 
would converge just one meter angle, they would also 
accommodate just one diopter ; to see a point at y^ of a 




Fig. 80. 



meter they would converge just three meter angles, and at 
the same time would accommodate three diopters, etc., 
thus showing how intimately the powers of convergence and 
accommodation are linked together, though it is possible 
to converge without accommodation (see Presbyopia) or to 
accommodate without convergence (paralysis of the in- 
terni). 

Far and Near Points of Convergence. — Just as we have 
a far and a near point of accommodation, we also have a far 
and a near point of convergence. The far point of conver- 
gence is the point to which the visual lines are directed 
when convergence is at rest, or at a minimum. The near 



ANGLE GAMMA. 85 

point of convergence is the point to which the visual lines 
are directed when the eyes are turned inward to their 
utmost degree. 

Infinity, or parallelism, is the position of the visual lines 
in the standard eyes in a state of rest (E oo, in Fig. 80). 
Visual lines that diverge in a state of rest can only meet by 
being projected backward, and, therefore, meet at an imag- 
inary point behind the eyes (N, in Fig. 80) ; convergence is 
then spoken of as negative, or minus ( — ). 

If the visual lines meet in a state of rest, then conver- 
gence is spoken of as positive ( + ). 

The amplitude of convergence is the distance measured 
from the far point to the near point of convergence, and is 




Fig. 81. 

represented by the greatest number of meter angles of con- 
vergence which the eyes can exert. 

Angle Gamma. — An understanding of what is known as 
the angle gamma is important, that the observer may 
understand and appreciate the real or apparent position of 
the eyes when looking at a near or distant point. Figure 8 1 
shows the line O A (optic axis) and the optic center, or 
nodal point (X), is situated on this line in the posterior part 
of the crystalline lens. The line V M is really a secondary 
axis to this dioptric system of the eye, and unites the object 
(V) with the fovea centralis at M ; this line is known as the 



86 



REFRACTION AND HOW TO REFRACT. 



visual line. The angle formed by the visual line with the 
optic axis at the nodal point may be considered as the angle 



If the fovea centralis at M was situated on the optic axis 
at A, then the visual line and optic axis would coincide, 
and there would not be any angle gamma. 

In hyperopic and emmetropic eyes the outer extremity 

of the visual line lies 5, 7, 
or, in some instances, as 
much as 10 degrees to the 
nasal side of the optic axis 
(averaging about 5 de- 
grees), and is spoken of 
as positive, and given the 
plus sign. In some long 
myopic eyes, however, the 
outer extremity of the 
visual line may lie to the 
outer side of the optic 
axis, when it is spoken of 
as negative, and given the 
minus sign. 

To demonstrate the an- 
gle gamma, the patient is 
told to look at the point of a pencil or pen held in the 
hand of the surgeon, about 1 3 inches distant (A in Figs. 
82 and 83). If the angle gamma is positive, the eyes will 
appear divergent to the observer, who looks at the position 




Fig. 82. 



* This is not a perfectly correct statement, as the real angle gamma is the 
angle formed by the line of fixation V R with the optic axis, R being the 
center of rotation. The angle V N O and the angle V R O being so nearly 
equal, are, for all intents and purposes, considered as the same. 



ANGLE ALPHA. 



87 



of the poles of the corneas or centers of the pupils. (See 
Fig. 82.) 

If the angle gamma is negative the eyes will appear 
convergent — that is, they appear to converge to a point in 
front of the pencil. (See 
Fi 



, S3-) 




The amount of the 
angle gamma can be 
measured by using the 
arc of the perimeter held 
horizontally, the patient 
being placed in the same 
position as when having 
his field taken. To do 
this, while the eye fixes 
the central point, the 
surgeon passes a candle- 
flame along the arc until 
the catoptric image of 
the flame is seen at the 
center of the pupil ; this 
position of the candle- 
flame on the arc is noted in degrees, which is the size of the 
angle gamma. 

Angle Alpha. — This is the angle formed by the long 
axis of the corneal ellipse with the visual axis. In the 
consideration of this angle it must be remembered that the 
cornea resembles, in its central area, at least, an ellipsoid 
of revolution, with the shortest radius usually in the verti- 
cal meridian. The angle alpha is spoken of as positive 
when the outer extremity of the long axis of the cornea is 
to the outer side of the visual line, and negative when it 
is to the nasal side. 



Fig. 83. 



CHAPTER III. 

OPHTHALMOSCOPE. 

Direct and Indirect Method. 

Ophthalmoscope. — From o<pdaX<±oq, "eye," and (rxonelv, 
" to observe " or " view "; literally, " to view an eye." An 
instrument used for studying the media and interior of the 
eye. The pupil of an eye in health appears to an observer as 
black ; this is due to the fact that the observer's eye does 
not ordinarily intercept any of the rays of light which return 
from the eye. Rays of light entering an eye are returned 
toward their immediate source, and, therefore, if an observer 
wishes to see into or study the interior of an eye, he must 
have his own eye in the path of the returning rays. To 
accomplish this, the observer places a mirror in front of his 
eye, so that the reflected rays entering the eye are returned 
toward the mirror. There is an infinite variety of these in- 
struments in the market, but for the general student the 
modified instrument of Loring appears to meet with most 
favor. (See Fig. 84.) 

This has a concave mirror with a radius of curvature of 
40 cm., giving a principal focus, therefore, at 20 cm. The 
sight-hole is round and about 3^ mm. in diameter, cut 
through the glass ; this mirror can be tilted to an angle of 
25 degrees. As an improvement over such a mirror, and to 
take its place, the writer would recommend the mirror used 
on his own ophthalmoscope, which has a radius of curvature 
of 15 cm.; and the sight-hole, 2}4 mm. in diameter, is not 
cut through the glass, but is made by removing the quick- 

88 



OPHTHALMOSCOPE. 



89 



silver. The glass at the sight-hole gives additional reflect- 
ing surface, and at the same time does away with much 
annoying aberration which results when the glass is per- 
forated. 




BACK 



Fig. 84. 



The small sight-hole is an advantage, also, in looking 
into small pupils. The mirror, oblong in shape, 18 by 33 
mm., is secured at the center of its ends, by two elevated 
screws, to a hollow disc 4^ cm. in diameter, in which is a 
revolving milled wheel, containing small spheres, each 
about 6 mm. in diameter. The series of spheres ranges 
8 



9 o 



REFRACTION AND HOW TO REFRACT. 



from — i D. to —8 D., and from + 1 D. to -f 7 D. The 
central aperture does not contain a lens, but is left open. 

When it is desirable to use any lens stronger than' — 8 
D. or -f-7 D., there is an additional quadrant, which can be 
superimposed and turned into place at the sight-hole ; it 
contains four lenses, — 0.50 D. and — 16 D., also +0.50 
D. and -f-16 D. With this quadrant and the spheres in 
the milled wheel, any spheric combination can be made 
from zero to — 24 D. or to + 2 3 D. An index below the 
sight-hole of the instrument records the strength of lens 




Fig. 85. 



that may be in use ; minus lenses are usually marked in red 
and plus lenses in white. 

How to Use the Ophthalmoscope. — There are two ways 
or methods by which the ophthalmoscope may be used — 
the direct and the indirect. 

The Direct Method (see Fig. 85). — Proficiency with 
the ophthalmoscope does not come except from long and 
constant practice, and several important matters should 
receive very careful attention before the student attempts to 
study the interior of an eye. 



OPHTHALMOSCOPE. 9 1 

The Room. — This should be darkened by drawing the 
shades or closing the blinds ; the darker the room, the better. 

The Light. — This should be steady, clear, and bright ; 
a good lamp is suitable, but an Argand burner gives more 
intense light, and is to be preferred, especially if it is placed 
on an extension bracket that can be raised or lowered 
and is capable of lateral movement. 

Position of Light and Patient. — The light should be 
several inches to one side and back of the patient, and on a 
level with the patient's ear, so as to illuminate the outer half 
of the eyelashes of the eye to be examined ; it may even 
be well to have the tip of the patient's nose illuminated. 

The patient should be seated in a comfortable chair 
(without arms), and is instructed to look straight ahead into 
vacancy, or at a fixed object if necessary, and is only to 
change the direction of his vision when told to do so. 
Under no circumstances should the patient be allowed to 
look at a light, as this will contract the pupil. 

For the beginner, it may be well to dilate the patient's 
pupil with a solution of cocain or homatropin. The 
student, however, should learn as soon as possible to see 
into an eye without the aid of a mydriatic, as many patients 
seriously object to the slight inconvenience that results 
from the drugs mentioned. 

The Observer. — If the observer has any decided refrac- 
tive error, he should wear his correcting glasses ; the reason 
for this will be explained later. The observer should be 
seated at the side of the patient corresponding to the eye 
he is to examine. Examining the right eye, the observer 
should be on the patient's right ; if the left eye, then on 
the patient's left. 

When examining the right eye, the ophthalmoscope is 
held in the right hand, before the right eye ; and in the left 



92 REFRACTION AND HOW TO REFRACT. 

hand, and before the left eye, when examining the left eye. 
The surgeon's eye should be a little higher than the patient's. 
Patient and observer should keep both eyes open. The 
one exception to this is when the patient has a squint, 
when it will be necessary for him to cover the eye not being 
examined, and in this way the eye under observation will look 
straight ahead. 

The surgeon holds the ophthalmoscope perpendicularly, 
so that the sight-hole in the mirror is directly opposite to 
his pupil and close to his eye. The side of the instrument 
rests on the side of his nose or the upper margin is in the 
hollow of the brow. The mirror is tilted toward the light. 
The surgeon's elbow should be at his side, and not form an 
angle with his body. 

With these several details carefully executed, the surgeon 
begins his examination at a distance of about 25 or 30 cm., 
never closer ; and at this distance he reflects the light from 
the mirror into the eye, and observes a " red glare," which 
occupies the previously black pupil. This is called the 
" reflex," and is due to the reflection from the choroidal 
coat of the eye. The color of the reflex varies with the size 
of the pupil, transparency of the media, the refraction, and 
the amount of pigment in the eye-ground. 

Having obtained the " reflex," it will be well for the be- 
ginner to practise keeping the reflected light upon the pupil 
by changing his distance, approaching the eye as close as 
an inch or two ; this must be done slowly, and not with a 
rush. 

What the Observer Sees. — Having learned to keep the 
light on the pupil, the next thing is to study the transparency 
of the media — i. e., to find out if there is any interference 
with the free entrance and exit of the reflected rays, such 
as would be caused by opacities in the cornea, lens, lens 



OPHTHALMOSCOPE. 93 

capsule, or vitreous ; and, if present, to note their character 
and exact location, whether on the visual axis or to one 
side, etc. The next objective points will be mentioned 
individually, and with the idea of systematizing the study. 

The Optic Nerve. — Also called the disc or nerve head 
or papilla. 

Color of the Optic Disc. — This has been described as 
resembling in color the marrow of a healthy bone, or the 
pink of a shell, etc. ; yet this is not by any means a true 
statement or description, as the apparent color of the nerve 
is controlled in great part by the surrounding eye-ground — 
whether this is heavily pigmented or but slightly so, or 
whether there is an absence of pigment, as in the albino. 
The student should be ready to make allowances for these 
contrasts. 

The shape of the disc varies : it may appear round, oval, 
or even irregular in outline. Usually it is a vertical oval. 

The vessels on the disc which carry the blood to and 
from the retina are not of the same caliber, nor do they 
have the same curves and branches in all eyes or in the 
same pair of eyes. The central artery may be single or 
double (if it has branched in the nerve before entering the 
eye), and enters the eye at the nasal side of the center of 
the disc. 

Approximating the central artery on its temporal side is 
the retinal vein, which may also be double. The relative 
normal proportion in size between arteries and veins is 
generally recognized as -about two to three. The veins are 
usually recognized by their larger size and darker color. 
At or near the center of the disc is often seen a depression, 
known as the physiologic cup ; this may be shallow or 
deep ; it may have shelving or abrupt edges ; it may even 
be funnel-shaped. 



94 REFRACTION AND HOW TO REFRACT. 

At the bottom of the cupping is frequently seen a gray 
stippling, the membrana cribrosa ; openings in the sclera for 
the passage of the transparent optic nerve-fibers which go to 
form the retina. Surrounding the disc proper is often seen 
a narrow white ring ; this is sclera, and is known as the 
scleral ring. Just outside of this ring is frequently seen a 
ring of pigment ; this is called the choroidal ring. In many 
cases the choroidal ring is not complete, the pigment being 
quite irregular, or possibly there may be just one large 
mass of pigment to one side of the disc ; this is not patho- 
logic. 

The retinal arteries and veins, while possessing many 
anomalies, and while occasionally an artery and vein are 
seen to twine around each other, usually pursue a uniform 
course up and down from the disc, and are named accord- 
ingly — i. c, upper nasal vein and artery ; upper temporal 
vein and artery ; lower nasal artery and vein ; lower tem- 
poral artery and vein. 

The retina itself, in health being transparent, is not seen. 
The fovea centralis, occupying the center of the macular 
region, is about two discs' diameter to the temporal side 
of the disc and slightly below the horizontal meridian. 
The fovea is recognized because it is a depression, and its 
edges give a reflex ; it is very small, and appears as a bright 
spot one or two mm. in diameter. The " macular region " 
is the part of the eye-ground immediately surrounding the 
fovea ; it contains minute capillaries, but it is impossible, in 
healthy eyes, to recognize them with the ophthalmoscope. 

The Choroid. — This is distinguished by the character of 
its circulation, the vessels being large, numerous, and flat- 
tened, and without the light streak which characterizes the 
retinal vessels. Pigment areas between the vessels are also 
diagnostic of this tunic. The choroidal circulation is best 



OPHTHALMOSCOPE. 



95 



studied in the blond or albino, and may be seen in many 
eyes toward the periphery of the eye-ground. 

In the foregoing description of the use of the ophthal- 
moscope, etc., it is presumed that the instrument has been 
used without any lens in position, and that the observer's 
eye and the eye under examination are healthy emmetropic 
eyes with the accommodation at rest. Figure 86 shows the 
position of the light, L, the ophthalmoscope, the examiner's 
and the examined eye under these conditions. 

The divergent rays from the light (L) are reflected con- 




Fig. 86. 



vergently from the concave mirror, and focusing in the vitre- 
ous, they cross and form an area of illumination on the 
retina at IF. The retina, situated at the principal focus of 
the dioptric media, naturally projects out from its indi- 
vidual points rays of light which are parallel as they leave 
the eye; some of these pass through the, sight-hole of the 
mirror and meet upon the retina of the observer's emme- 
tropic eye. 

There are two very important points which must be 



9 6 



REFRACTION AND HOW TO REFRACT. 



considered when using the ophthalmoscope in the direct 
method : one is the direction which the rays of light take 
as they leave the eye under examination, and the other is 
for the observer to keep his own eye emmetropic ; in other 
words, the observer wearing his correcting glasses should 
not accommodate. 

Figure 87 shows that rays of light passing out of an eye 
divergently must be made parallel, so as to focus upon the 
surgeon's own retina (emmetropic), and to do this it is 




Fig. 87. — T B indicate .points at the edge of the disc from which rays pass 
out of the eye divergently in the direction T / B 7 , T / B 7 , T 7 B 7 , and being 
received by the observer's eye, are projected backward, forming an erect 
magnified image at T 7/ B 7/ . This image is not so large as that seen when 
looking into a myopic eye. (Fig. 88.) 



necessary to turn a plus lens in front of the sight-hole of 
the ophthalmoscope ; the strength of the convex lens thus 
employed, other things being normal, is the amount of the 
refractive error of the eye being examined. 

Figure 88 shows rays of light passing out of an eye 
convergently, and to have them parallel, so as to focus 
upon his own retina (emmetropic), it is necessary to turn a 
concave lens in front of the sight-hole of the ophthalmo- 



OPHTHALMOSCOPE. 



97 



scope ; the strength of the concave lens thus employed, 
other things being normal, is the amount of the refractive 
error of the eye under examination. 

The Observer's Accommodation. — It has already been 
stated that, when using the ophthalmoscope, the observer 
should wear any necessary correcting lenses. If the ob- 
server has a refractive error and does not wear his glasses, 
he must deduct this amount from the lens used in the 
ophthalmoscope. If he has two diopters of hyperopia 




FlG. 88. — T B indicate points at the edge of the disc from which rays pass 
out of the eye convergently in the direction T' B / , and, being received by 
the observer's eye, are projected backward, forming an erect magnified 
image at T" W . This image is much larger than that seen when look- 
ing into the hyperopic eye. (Fig. 87.) 



himself, and the lens used in the ophthalmoscope is plus 
four diopters, then the eye under examination has only two 
diopters. It is not unusual for beginners to see the eye- 
ground (disc) in hyperopic eyes with a strong concave 
lens ; this is due to the fact that they accommodate. Prac- 
tice will overcome this habit, and it should be mastered as 
soon as possible. There are several ways of doing this : 
one is to begin the examination at a distance of 30 or 40 
9 



g8 REFRACTION AND HOW TO REFRACT. 

cm. from the eye, with both eyes open, and to gradually 
approach the eye as close as 3 cm., imagining all the time 
that one is looking for some remote point ; otherwise, if one 
begins the examination close to the eye, and imagines he is 
going to see an object- about an inch away, he will most 
invariably accommodate several diopters, with the result 
that he turns a strong concave lens in front of the sight- 
hole of the ophthalmoscope to neutralize his accommodation. 

This explains how so many beginners diagnose all cases 
of hyperopia as myopia. An excellent way to learn to 
relax the accommodation is to practise reading fine print 
at a distance of about thirteen inches through a pair of 
plus three lenses, placed before the surgeon's emmetropic 
eyes. Another good way to learn to relax the accommo- 
dation is to practise on one of the many schematic eyes 
found in the shops. (Fig. 143.) 

Size of the Image of the Eye-ground (Figs. 87 and 
88). — In concluding the subject of tiie direct method of 
examination it may be interesting to note the apparent size 
of the image of the eye-ground, which, it must be remem- 
bered, is virtual, erect, and enlarged ; in fact, it seems to 
be at some distance behind the eye, and if the student has 
paid close attention to the study of images as formed by 
convex lenses, detailed in chapter 1, he need not have any 
difficulty in appreciating these facts. 

The optic disc of an emmetropic eye, as seen through 
the ophthalmoscope, appears to be about 25 mm. in diam- 
eter, and about 250 mm. away. The retina of the emme- 
tropic eye is about 1 5 mm. from its nodal point ; then the 
actual size of the emmetropic disc is -^fy of 25, or -|, or 
1.5 mm. ; then 15 is to 250 as 1.5 is to 25", or 16.6 — the 
magnification, in other words, when the emmetropic disc is 
observed, it appears about 16.6 times larger than it actually is. 



OPHTHALMOSCOPE. 



99 



The Indirect Method (see Fig. 89). — Practising this 
method, the observer sees a larger part of the eye-ground 




Fig. 89. 



at one time, but it is not so perfect in detail nor is it mag- 
nified to the same extent as in the direct method. The 
observer does not have to get so close to his patient, which 
is a decided advantage in some clinical cases. Unfortun- 
ately, as a preliminary 
step, it is often neces- 
sary to dilate the pu- 
pil. In addition to 
the ophthalmoscope, 
there is also required 
a convex lens of 
known strength and 
large aperture ; the 

one which comes in 

1 • , , FlG - 9°- 

the case with the 

scope is usually too small and too strong for general use. 
The writer prefers his plus 13 D. with metal rim and conve- 
nient handle, shown in figure 90 (reduced one-third in size). 




IOO REFRACTION AND HOW TO REFRACT. 

This is held at about three inches in front of the eye 
under examination, the observer resting his little and ring 
fingers on the temple of the patient. The light may be 
over the patient's head, or to the side corresponding to the 
eye under examination, the patient being instructed to look 
with both eyes open toward the surgeon's right ear when 
the right eye is being examined, and toward the surgeon's 
left ear when the left eye is examined. 

With a -]- 4 D. in the ophthalmoscope held close to his 
eye, the surgeon seats himself in front of the patient at 
about sixteen inches distant, and reflects the light through 
the condensing lens into the patient's eye, and then 
approaches or moves away from the eye until he recog- 
nizes clearly a retinal vessel or the disc ; he must remem- 
ber, however, that he is not looking into the eye, but is 
viewing an aerial image formed between the convex lens 
and the ophthalmoscope ; this image is not only inverted, 
but undergoes lateral inversion, so that the right side 
of the disc becomes the left side of the image, and vice 
versa ; the upper side of the disc becomes the lower side 
of the image, and vice versa. As the direct method gives 
an erect, virtual, and enlarged image, the indirect method 
produces an inverted, real, and small image. The principle 
of the direct method is similar to a simple microscope, and 
the indirect to a compound microscope. 

The size of the image depends upon the refraction of 
the eye and the distance of the convex lens from the eye 
under examination. In the standard eye this is always the 
same, no matter how far away from the eye the convex lens 
is held. To estimate the size of the image in the standard 
eye, all that is necessary to know is the principal focal dis- 
tance of the lens employed ; if a -f-13 D., then the image 
is formed at 75 mm. (three inches), and remembering that the 



OPHTHALMOSCOPE . 



IOI 



retina in the eye is 15 mm. back of the nodal point, the 
size of the image will be to the size of the disc (if that is 





Fig. 91. Fig. 92. 

Figs. 91 and 92. — DeZeng Luminous Ophthalmoscope. Two-thirds size. 



what is looked at) as their respective distances, or as 15 is 
to 75, which equals 5, the magnification. 



102 REFRACTION AND HOW TO REFRACT. 

The purpose of the +4 D. in the scope is to take the 
place of the eye-piece in the microscope, and, therefore, to 
magnify the image at the same time it relieves the observer's 
accommodation. In high myopia the +4 D. may be dis- 
pensed with. 

The Luminous Ophthalmoscope (Figs. 91 and 92). — 
DeZeng Patent (Model 1908). — This instrument is a com- 
bination of the Loring ophthalmoscope just described, with 
the addition of an electric light attachment. The mirror 
is somewhat different from the mirror on the Loring instru- 
ment. It is plane, circular in form, and 14 millimeters in 
diameter. The mirror is placed at an angle of 43 degrees. 
The handle of the instrument carries the electric wires to a 
small lamp, and between the lamp and the mirror is placed 
a very strong convex lens. The rays of light from the filament 
falling upon the convex lens are refracted very con ver gently, 
and after reflection from the mirror converge to a point one 
inch distant. (See Fig. 92.) 

This instrument is ideal for both the direct and indirect 
method and has the following points of merit: The mirror 
and light are stationary, thus giving the observer any liberty 
of movement necessary without any loss of reflection from the 
mirror; the mirror never requires any tilting; the brilliancy 
or intensity of the illumination at the fundus, by virtue of the 
light being so close to the mirror, far exceeds that of the 
nonluminous instrument; for the same reason the size of the 
retinal illumination is made about five times larger than that 
by the old style instrument. The heat from the electric 
lamp (2 J volts, I ampere) is infinitesimal. 



CHAPTER IV. 

EMMETROPIA.— HYPEROPIA.— MYOPIA. 

Emmetropic 

Emmetropia (&», ''in"; fiirpov, "measure"; «5#, 
" eye ") literally means an eye in measure, or an eye which 
has reached that stage of development where parallel rays 
of light will be focused on its retina without any effort of 
accommodation. As the emmetropic eye is the ophthal- 
mologist's ideal unit of measurement or goal in refraction, 
the beginner should know this form of eye thoroughly, so 
that he may recognize any departure from this standard 
condition. The emmetropic eye may be described in vari- 
ous ways, and while these descriptions may appear like 
repetitions, they are given for purposes of illustration : 

The standard or schematic eye : Authorities differ some- 
what in the exact measurements of a schematic eye, but 
the one suggested by Helm- 
hoitz is certainly worthy of 
careful consideration. (See 

P- 59-) 

An emmetropic eye is one 

which, in a state of rest 

(without any effort of accom- Fig. 93. 

modation whatever), receives 

parallel rays of light exactly at a focus upon its fovea. 

(See Fig. 93.) 

An emmetropic eye, therefore, is one which, in state of 
rest, emits parallel rays of light. (See Fig. 93.) 

103 




104 



REFRACTION AND HOW TO REFRACT. 




An emmetropic eye is one 
whose fovea is situated exactly 
at the principal focus of its re- 
fractive system. (See Fig. 93.) 

An emmetropic eye is one 
the vision of which, in a state 
of rest, is adapted for infinity. 

An emmetropic eye is one 
which has its near point consist- 
ent with its age. (See p. 70.) 

An emmetropic eye is one 
which does not develop pres- 
byopic symptoms until forty- 
five or fifty years of age. (See 
p. 272.) 

An emmetropic eye, in con- 
tradistinction to a myopic eye 
(see p. 1 1 5), is spoken of as 
a healthy eye, or one which 
shows the least amount of irri- 
tation in its choroid and retina. 

Because we refer to Helm- 
holtz's schematic eye as an em- 
metropic eye, it will not do to 
say that all eyes that measure 
just 23 mm. in their antero- 
posterior diameter are emme- 
tropic (Fig. 94) ; for while an 



FlG. 94. — I. Emmet'ropia. 2. Myopia 
due to a strong lens. 3. Hyperopia 
due to a weak lens. 4. Myopia due 
to a short radius of curvature of cornea. 
5. Hyperopia due to a long radius of 
curvature of cornea. The anteropos- 
terior diameter of all these eyes is just 
23 mm. 



AMETROPIA. IO5 

eye may be just 23 mm. in length, it may have its refractive 
system stronger or weaker than is consistent with its length, 
making it, if stronger, a myopic or long eye, and, if weaker, 
a short or hyperopic eye. An eye, to be emmetropic, 
therefore, no matter what its length, must have its refractive 
apparatus of just such strength that, in a state of rest, the 
principal focus will coincide exactly with the cones at the 
fovea. (Fig. 93.) 

Ametropia. 

Ametropia (a priv. ; fiirpov, " a measure" ; d<pts, " sight") 
literally means " an eye out of measure." An ametropic 
eye is one which, in a state of rest, does not form a distinct 
image of distant objects upon its retina. An ametropic 
eye may be denned as one which, in a state of rest, does 
not focus parallel rays of light upon its fovea. An eye 
which is not emmetropic is ametropic. There are two 
forms of ametropia — axial and curvature ametropia. 

Axial ametropia is the condition in which the dioptric 
apparatus refracts equally in all meridians, but the retina 
of the eye, when at rest, is either closer to, or further away 
from, the nodal point than the principal focus. (See Figs. 
95 and 97.) The refraction is measured on the length of 
the anteroposterior axis of the eye ; hence its name, axial 
ametropia. 

Curvature ametropia, in contradistinction to axial ame- 
tropia, is the condition in which the dioptric apparatus does 
not refract equally in all meridians, and with the result that 
there is no focusing of all the rays at any one point ; or 
curvature ametropia may be considered as that condition 
in which parallel rays of light entering an eye have two 
focal planes for two principal meridians at right angles to 
each other. Curvature ametropia is commonly spoken of 
as astigmatism. (Sec Chap, v.) 



106 REFRACTION AND HOW TO REFRACT. 

Varieties of Axial Ametropia. — Axial ametropia is of 
two forms : one in which the eye has its fovea closer to the 
dioptric apparatus than its principal focus (see Fig. 95), 
known as the hyperopic, short, or flat eye ; and the other 
form of the eye in which the fovea is further away than its 
principal focus, known as the myopic or long eye. (See 

Fig. 97) 

Hyperopia or Hypermetropia. 

Hyperopia (Mp, "over"; a><p, "eye") literally means 
an eye which does not equal the standard condition, or an 
eye which is less than the standard measurement. Hyper- 
opia is often abbreviated H. About twenty per cent, 
of all eyes have simple hyperopia. The hyperopic eye is 
spoken of as far-sighted, and the condition as one of far- 
sightedness. The hyperopic eye may be described in many 
different ways : 

1. The " natural eye," or "the eye of nature." 

2. The "short eye." This term is used on account of 
its fovea lying closer to the dioptric apparatus than the 
principal focus. 

3. Parallel rays of light passing into a hyperopic eye in 
a state of rest fall upon its retina or fovea before they focus. 
(See Fig. 71.) 

4. Rays of light from the fovea of a hyperopic eye in a 
state of rest pass out divergently (see Fig. 95), and the 
condition is equivalent to a convex lens refracting rays of 
light which proceed from a point closer to the lens than its 
principal focus. (See Fig. 39.) 

5. A hyperopic eye is one which, in a state of rest, can 
receive only convergent rays of light at a focus upon its 
fovea (Fig. 95) ; therefore, to repeat : the hyperopic eye, in 
a state of rest, emits divergent rays and receives convergent 
rays at a focus upon its fovea. 



HYPEROPIA. 107 

6. As convergent rays are not found in nature, and are, 
therefore, artificial, a hyperopic eye is one which, in a state 
of rest, requires a convex lens to focus parallel rays of light 
on its fovea. (See Fig. 96.) 

~. A hyperopic eye is one which must accommodate for 
infinity, and, in fact, for all distances ; in other words, a 
hyperopic eye in use is in a constant state of accom- 
modation. 

8. A hyperopic eye having to use some of its accommo- 
dative power for infinity, must, in consequence, have its 
near point removed beyond that of an emmetropic eye of 
corresponding age. (See p. J2.) 

9. From the description contained in 3, it follows that 




Fig. 95. Fig. 96. 

the far point of a hyperopic eye in a state of rest is negative 
( — ), and is found by projecting the divergent rays back- 
ward to a point behind the retina. (See Fig. 95.) 

10. From the description contained in 6, and the descrip- 
tion of accommodation on page 67, it is natural to find the 
retina and choroid of many hyperopic eyes in a state of 
irritation. 

11. From the description contained in 6 and 7, and on 
page 273, it follows that symptoms of presbyopia manifest 
themselves earlier in hyperopic than in any other form of 
eyes. 

12. From the description contained in 5, — and this may 



108 REFRACTION AND HOW TO REFRACT. 

appear like repetition), it follows that a hyperopic eye will 
accept a plus glass for distant vision. (See Fig. 96.) 

13. From 6 it is evident that the circular fibers of the 
ciliary muscle must become highly developed ; much more 
so than the longitudinal fibers. Microscopically, a section 
of the ciliary muscle on this account will bear evidence of 
the character of the eye from which it came. 

Causes of Hyperopia. — It is a well-known fact that the 
eyes of the new-born are, with comparatively few excep- 
tions, hyperopic ; such eyes are supposed to grow in their 
anteroposterior diameter, and at adolescence to reach that 
stage of development called emmetropia. It is also a well- 
known fact that this ideal condition of emmetropia is very 
rarely attained, the length of the eyeball not increasing in 
proportion to the strength of its refractive system. 

Eyes may approximate the emmetropic condition, but 
very seldom remain so, passing into the condition in which 
the fovea lies beyond the principal focus, becoming what is 
known as long, or myopic. 

A standard eye may be made hyperopic by removing 
its lens ; the condition following cataract extraction. (See 
Fig. 174.) 

An eye may possibly become hyperopic in old age, from 
flattening of the lens due to sclerosis of its fibers. 

Any disease which will cause a flattening of the cornea 
in a standard eye will produce hyperopia. 

A diminution in the index of refraction of the media 
of the standard eye will produce hyperopia. 

Subdivisions of Hyperopia. — For purposes of study 
hyperopia has been divided into six classes or forms : 

I. Facultative hyperopia (abbreviated Hf.) is a condi- 
tion of the eye in which the patient can overcome the error 
by using his accommodation. It is a condition of early 



HYPEROPIA. 109 

life, and is voluntary. The patient can see clearly, with or 
without a convex glass. 

2. Absolute hyperopia (abbreviated Ha.). — This is hy- 
peropia that can not be overcome by the accommodative 
effort. It is generally a condition of old age, and is invol- 
untary ; facultative hyperopia in youth becomes absolute in 
old age. Old age, in fact, may develop each variety except 
latent hyperopia. Absolute hyperopia exists whenever the 
defect is of so high a degree that it can not be overcome 
by the accommodation or when the accommodative power 
itself is gone. 

3. Relative hyperopia (abbreviated Hr.) is where ac- 
commodation is assisted in its efforts by the internal recti 
muscles ; in other words, the eyes squint inward. 

4. Manifest hyperopia (abbreviated Hm.) is repre- 
sented by the strongest convex lens through which an eye 
can maintain distinct distant vision. Manifest hyperopia, 
therefore, includes facultative and absolute. 

5. Latent hyperopia (abbreviated HI.) is the amount 
of hyperopia which an eye retains when a plus lens is 
placed in front of it. Or latent hyperopia is the difference 
between the manifest hyperopia and that lens which an eye 
would select if its accommodation was put at rest with a 
cycloplegic (atropin). For example, an eye accepts a 
+ I.25 S. as its manifest H., and, when atropin is instilled, 
would accept +2.75 S. for the same distant vision ; then 
the difference between the manifest -{-1.25 S. and +2.75 
S. (the total) is -(-1.50 S., which is the latent hyper- 
opia. 

6. Total hyperopia (abbreviated Ht.) is the full amount 
of the hyperopia ; or is represented by the strongest glass 
which an eye will accept, and have clear, distinct vision 
when in a state of rest. 



IIO REFRACTION AND HOW TO REFRACT. 

Symptoms and Signs of Hyperopia. — These are many 
and various ; the principal one, however, and the one that 
generally causes the patient to seek relief, is headache. 
Headache caused by the eyes is usually frontal, and is 
denominated " brow ache"; it may be frontotemporal ; 
the pain or discomfort starting in or back of the eyes may 
extend to the occiput or all over the head, and be accom- 
panied with all kinds of nervous manifestations. The most 
characteristic distinguishing feature of ocular headache is 
that it comes on while using the eyes, and gradually grows 
worse as the use of the eyes is persisted in ; and, likewise, 
the headache gradually ceases after a few minutes' or hours', 
rest of the eyes. Vertex headache, or a feeling of weight 
on the top of the head, has been preempted by the gyne- 
cologist, and is not usually classed as ocular. The ciliary 
muscle being the prime factor in causing the headaches, 
the writer feels justified in calling it the " headache mus- 
cle." " Sick headaches " are largely due to eye-strain. 
Various functional disorders, such as dyspepsia, constipa- 
tion, biliousness, lithemia, chorea, convulsions, epileptoid 
diseases, hysteria, melancholia, etc., are, according to some 
few authorities, attributable to this condition. See Asthen- 
opia, page 219. 

Blepharitis marginalis, styes, and conjunctivitis are 
frequently present, and in truth the hyperopic eye on 
this account can often be diagnosed in public outside 
of the surgeon's office. A feeling as of sand in the eyes, 
ocular pains or postocular discomfort, a dryness of the 
lids, as if they would stick to the eyeballs, are common 
complaints, and part of the conjunctivitis. Other patients 
have their eyes filling with tears (epiphora) as soon as they 
begin reading, etc. A drowsiness or desire to sleep often 
comes on after or during forced accommodation. 



HYPEROPIA. I I I 

Congestion of the choroid and retina, as evidenced by the 
ophthalmoscope, often go together with the blepharitis and 
conjunctivitis. 

The patient complains that the print blurs or becomes dim 
after reading, and this is especially apt to occur by artificial 
light. When the "blur" comes on, he has to stop and 
rub his eyes or bathe them ; and then, with additional light, 
he is able to continue the reading for a short time longer, 
when the blur again returns and the effort must be given up. 
Strong light stimulates the accommodation. The " hyper- 
opic blur" is nothing more or less than a relaxation of the 
accommodation. 

In children hyperopia sometimes simulates myopia, from 
the fact that the child in reading holds the print very close 
to the eyes. He does this in order to get a larger retinal 
image and to relieve his accommodation ; the retinal image 
is not clear, and the child has to read slowly ; the retinal 
image is composed mostly of diffusion circles. The child 
holds the print close to his eyes to avoid using his total 
accommodation, which he might have to do if he held the 
print at a respectable distance. 

He also calls into play the orbicularis palpebrarum, and 
narrows the palpebral fissure, looking through a stenopeic 
slit, as it were. These cases of simulated myopia can be 
quickly diagnosed by : 

1. The narrow palpebral fissure during the act of reading, 
and reading very slowly, as each letter has to be studied. 

2. The fact that very few children have myopia. 

3. The comparatively good distant vision, as a rule, 
which myopes never have, unless the myopia is of very 
small amount. 

4. The ophthalmoscope. 

The beginner in ophthalmology should be on his guard 



I 1 2 REFRACTION AND HOW TO REFRACT. 

for these " pseudo-myopias," and not be guilty of putting 
concave lenses on hyperopic eyes. 

Diagnosis of Hyperopia. — This form of ametropia may 
be recognized in many ways : 

i. Blepharitis marginalis, if present, is generally due to 
hyperopia. 

2. Hyperopic eyes are said to be small, and to have 
small pupils, which facts are generally confirmed ; but 
myopic eyes sometimes appear small, and have small pupils 
also. 

3. A narrow face and short interpupillary distance are 
quite indicative of hyperopia, but these indexes are not 
infallible. 

4. A child with one eye turned inward toward the nose 
(convergent squint) has hyperopic eyes, as a rule ; the 
hyperopia generally not being of the same amount in the 
two eyes, the squinting eye usually being the more 
hyperopic. 

5. It has been authoritatively stated that light-colored 
irises are seen in hyperopic eyes and dark irises are to be 
found in myopic eyes, and yet this is not always correct. 
German students, with their blue irises, will average from 
50 per cent, to 60 per cent, of myopia. 

6. Hyperopic eyes, with few exceptions, have excellent 
distant vision : often -^j-, or even better. The student should 
be on his guard for this, and not imagine, because a 
patient has — vision, that he is emmetropic ; on the con- 
trary, hyperopic eyes accommodate for distance, and obtain 
this acute vision by effort. 

7. The patient gives a history of accommodative asthe- 
nopia, with or without headaches coming on during or 
after the use of the eyes. 

8. The distant vision of a hyperopic eye may remain 



myopia. i 1 3 

unchanged or may be improved with the addition of a 
convex lens, which latter would be impossible in emme- 
tropia and myopia. 

9. The near point of a hyperopic eye without glasses lies 
beyond that of an emmetropic eye for a corresponding age. 

10. A hyperopic eye can see fine print clearly through a 
convex lens at a greater distance than its principal focus, 
which would not be the case in any other form of eye. 

Other tests for determining hyperopia are with (11) the 
ophthalmoscope, (12) the retinoscope, (13) Scheiner's test, 
(14) Thomson's ametrometer, and (15) the cobalt-blue 
glass test, commonly spoken of as the chromo-aberration 
test. These tests are described in the text. 

Myopia. 
Myopia (fiueiv t "to close"; tu<p, "eye") means, liter- 
ally, "to close the eye," and this origin of the name has 
arisen from the fact that many long eyes (myopic) squint 
the eyelids together when they endeavor to see beyond 
their far point. Brachymetropia is another name occasionally 
mentioned for the same kind of eye. Myopia is abbreviated 
M. About 1.5 per cent, of all eyes have simple myopia. 
The myopic eye is spoken 
of as near-sighted, and the 
condition as one of near- 
sightedness. The myopic 
eye may be described in 
many different ways : 

1. The long eye. The 
origin of this name is 
purely anatomic, the fovea lying beyond the principal focus 
of the refracting system. (See Fig. 97.) 

2. Parallel rays of light entering a myopic eye focus in 

10 




97- 



I 14 REFRACTION AND HOW TO REFRACT. 

the vitreous humor before they can reach the fovea. (See 

Fig- 97-) 

3. Rays of light from the fovea of a myopic eye pass out 
of the eye convergently (see Figs. 72 and 98), focusing at 




Fig. 98. 

some point inside of infinity. The refractive condition of 
a myopic eye is similar or equivalent to a convex lens 
refracting rays of light which proceed from some point 
further away than its principal focus. (See Fig. 37.) The 
nearer the emergent rays of light focus to the eye (in a 
state of repose), the longer the eye ; and the further away 
the emergent rays focus from the eye, the nearer the eye 
approaches to emmetropia, or normal length. 

4. A myopic eye is one which receives rays of light 
which diverge from some point closer than six meters at a 

focus on its fovea and which 
emits convergent rays. (See 
Fig. 37, and also description 
of conjugate foci.) 

5. As parallel rays can not 

focus on the fovea of a 

FlG 99 myopic eye, it is necessary to 

give parallel rays entering the 

eye a certain amount of divergence, so as to place the focus 

at the fovea ; and to accomplish this, a concave lens must 

be used. (See Fig. 99.) A myopic eye, therefore, is one 




MYOPIA. I I 5 

which requires a concave lens to improve distant vision. 
(See Fig. 99.) 

6. A myopic eye is one whose distant vision is made 
worse by the addition of a convex lens. 

~ . A myopic eye is one which does not accommodate for 
distance. 

8. A myopic eye having a refracting system stronger 
than is consistent with its length, or vice versa, greater 
length than is consistent with its dioptric system, naturally 
does not use any accommodation except for points inside of 
its punctum remotum, and with the result that its ampli- 
tude of accommodation is used near by ; consequently, a 
myopic eye is one which has a near point closer than an 
emmetropic eye of corresponding age. (See p. 73.) 

9. From the description contained in 3 it follows that the 
far point of a myopic eye is positive ( + ). 

10. From the description contained in 3 and 7, it also 
follows that the myopic eye does not develop presbyopic 
symptoms until late in life. 

11. From 6 and 9 it follows that the circular fibers of 
the ciliary muscle are not used to the same extent in a 
myopic eye as in the emmetropic and especially in the 
hyperopic eye. Microscopically, a section of a ciliary 
muscle on this account will bear evidence of the character 
of the eye from which it came, and have the longitudinal 
fibers more in evidence. In some very long myopic eyes 
there may not be any circular fibers recognized. 

1 2. Eyes in which the myopia is progressive are spoken 
of as " sick eyes." 

Causes of Myopia. — Any disease or injury which will 
so alter the refracting system of an eye that parallel rays 
must focus in front of the fovea will produce the form of 
eye known as long or myopic. This may be brought about 



I 1 6 REFRACTION AND HOW TO REFRACT. 

in different ways : A shortening in the radius of curvature 
of the cornea, such as comes with conic cornea and staphy- 
loma of the cornea ; an increase in the refractive power 
of the lens from swelling, as often precedes cataract, and 
is spoken of as " false " second sight ; cyclitis and irido- 
cyclitis, which diseases cause a relaxation of the lens 
ligament, allowing the lens to assume a greater convexity ; 
or ciliary spasm may produce temporarily the same con- 
dition. 

Technically, however, myopia is quite universally under- 
stood to mean a permanent elongation of the visual axis 
of the eye beyond the principal focus of its refracting 
system. 

Heredity is certainly a predisposing factor to myopia, 
but this does not mean that the babe is necessarily born 
with long eyes. On the contrary, the eye is very likely 
hyperopic at birth, and what the child may inherit is weak 
eye tunics. Such eyes, when placed under strain or. what 
to them is overuse, soon become elongated. This may 
also be brought about or assisted by poor hygienic surround- 
ings, poor health, or develop after an attack of typhoid 
or one of the eruptive fevers. 

Three causes for the elongation of eyes have been brought 
forward by able authorities and expounded as theories, any 
one of which, or all three, may appear conspicuously in in- 
dividual cases. 

1. Anatomically, the size of the orbit and the broad 
face give a long interpupillary distance and cause excessive 
convergence (turning inward of the eyes) when the eyes fix 
at the near point. 

2. Mechanically, when the eyes are far apart and 
attempt to converge, the external recti muscles press upon 
the outer side of the globes, flattening the eyes laterally, 



MYOPIA. I 1 7 

with the result that the point of least resistance for the 
compressed contents of the globes is at the posterior pole 
of the eye, and here it is that the pressure shows itself, by 
an elongation of the eye backward in its anteroposterior 
diameter. This combination of the anatomic and mechanic 
theories may explain in great part the presence of myopia 
in the average German student or any broad-faced indi- 
vidual. 

3. The inflammatory theory is that a low grade of 
inflammation attacks the tunics of the eye, especially at 
the posterior pole, and is spoken of as macular chor- 
oiditis ; this is brought about by faulty use of the eyes, 
in the school or in the home, in a poor light or too 
glaring a light improperly placed, or by using the eyes 
with the head bent over the work so that the return circu- 
lation from the retina and choroid is interfered with. This 
inflammation or congestion of the tunics of the eye may 
be primary in itself or secondary to the anatomic and 
mechanic causes. Be this as it may, the conditions exist, 
and go to show more and more that myopia is actually 
acquired and not per se congenital. " The inherited con- 
genital anomalies of refraction, particularly astigmatism, are 
responsible for the myopic eye, by virtue of the pathologic 
changes they occasion in hard-worked eyes rather than any 
inherited predisposition to disease." (Risley, "School 
Hygiene.") 

Symptoms and Signs of Myopia. — While the myope 
may complain of headache and symptoms of accommodative 
asthenopia, yet the principal visual complaint will be the 
inability to see objects distinctly which lie beyond the far 
point. The myope's world of clear vision is limited to the 
distance of the far point, where the rays of light leaving his 
eye come to a focus. Every object situated beyond the far 



I 1 8 REFRACTION AND HOW TO REFRACT. 

point is blurred and indistinct, and the further the object 
from the far point, the more indistinct it becomes. The 
myopic child at school soon ranks high in the class, is fond 
of study, of books, music, or needlework, according to the 
sex. The myope, in other words, is usually literary in 
taste. Myopes. avoid out-of-door sports, such as foot-ball, 
base-ball, golf, etc. 

Diagnosis of Myopia. — This form of ametropia may be 
recognized in various ways : 

1. The prominent eyeball. This is not a positive sign of 
myopia, though this and other signs are mentioned for the 
reason that they are often present in the myopic condition. 

2. The broad face and (3) long interpupillary distance 
are quite significant of myopia, and yet the broadest face 
with longest interpupillary distance the writer ever saw 
was in a hyperopic subject. 

4. Divergent squint usually indicates myopia, and this 
condition is often brought about by an inability to converge, 
or one eye may be more myopic than its fellow, with the 
result that the more myopic eye turns out and soon be- 
comes amblyopic. 

5. It has been stated that myopic eyes usually have 
dark-colored irises, but this is often a fallacy, as is only too 
evident in the German student with his blue iris. 

The foregoing are but signs of myopia, and are recog- 
nized by inspection ; they should be looked for and care- 
fully estimated, and each given its due consideration. 
Subjective and objective symptoms are the true tests of 
myopia, and are as follows : 

6. Poor distant vision ; inability to see numbers on the 
houses across the street or on the same side of the street ; 
history of passing friends without speaking to them. The 
myope enjoys close work and takes little or no interest in 



Myopia. 1 19 

sports. A history, in other words, that is in keeping with 
a vision of short range. 

7. Good near vision ; ability to see the finest print or to 
thread the finest needle or do the finest embroidery. 

8. The near point is closer than that of an emmetropic 
eve of corresponding age. (See p. 73.) 

9. Distant vision is made worse by the addition of a 
convex lens. The writer prefers to teach the diagnosis of 
myopia in this way, and not to say that a concave lens 
will improve distant vision ; of course it will, but he does 
not want the student to put concave lenses before the 
eye of the young "pseudo-myope," referred to under 
Hyperopia. 

10. The far point is brought nearer by the addition of a 
convex lens. Objective methods of determining myopia 
are by means of the — 

11. Ophthalmoscope. 
Retinoscope. 
Scheiner's method. 
Thomson's ametrometer. 
Chromo-aberration test. 

Direct Ophthalmoscopy in Axial Ametropia. — Pro- 
ficiency in this method comes only by perseverance and 
long practice. It should not be employed to the exclusion 
of other and more exact methods. To estimate with the 
ophthalmoscope which lens is required to give an eye 
emmetropic vision, three very important facts should receive 
careful attention : 

1. The distance between the surgeon's and patient's eye. 

2. The surgeon's and patient's accommodation. 

3. The surgeon's own refractive error. 

First, the surgeon should have his eye as close to the 
patient's eye as possible, usually at 13 mm. ; this is the 



120 REFRACTION AND HOW TO REFRACT. 

anterior principal focus of the eye, and is the distance at 
which the patient will wear his glasses. 

Second, as already explained, the observer's and patient's 
accommodation should be in repose. The most difficult 
part for the student to learn is to relax his accommoda- 
tion. The ambitious student strains his accommodation 
(ciliary muscle) in his haste, and with the result that he 
thinks all eyes myopic and all eye-grounds as affected 
with " retinitis." 

Third, the surgeon, if not emmetropic, must wear any 
necessary correcting lenses ; otherwise, the lens in the 
ophthalmoscope will record his and the patient's error 
together, and deductions must be made accordingly. For 
instance, if the surgeon is hyperopic -f-2 S., and does not 
wear his glasses, and the ophthalmoscope records the fundus 
as seen clearly with -j- 5 S., this would mean that the patient 
had +3 S. (2 of the 5 S. being the surgeon's error); or 
if the fundus is seen without any lens in the ophthalmoscope, 
then the patient's error would be — 2 S. (the surgeon's 
+ 2 S. from leaving — 2 S.) ; or if the ophthalmoscope 
showed — 2 S., then the patient's error would be — 4 S. ; 
or if the ophthalmoscope registered +2 S., then the 
patient would be emmetropic, and this +2 S. is the sur- 
geon's error. 

Rules. — 1. When the surgeon and patient are both hy- 
peropic or both myopic, the surgeon must subtract his cor- 
rection from the lens which shows at the sight-hole in the 
ophthalmoscope. 

2. When the surgeon's eye is hyperopic or myopic, and 
the eye of the patient is the opposite, he must add his cor- 
rection to the lens at the sight-hole in the ophthalmoscope. 

With the foregoing details clearly in mind and carefully 
executed, the surgeon selects small vessels near the macula 



MYOPIA. I 2 I 

for his observations. If it is impossible to see these on 
account of the small pupil, then he will have to observe the 
larger vessels at the disc (nerve-head, or papilla). 

Whenever the vessels in the macular region are seen 
clearly with one and the same glass in the ophthalmo- 
scope, the refractive error can be approximated as one of 
axial ametropia, and every three diopters, plus or minus, or 
any multiple of three diopters, represent very closely one 
millimeter of lengthening or shortening of the anteropos- 
terior diameter of the eye. For example, any eye that 
tikes a plus 3 S. to make it emmetropic is just 1 mm. too 
short ; any eve that takes a minus 3 S. to make it emme- 
tropic is about I mm. too long. It will be observed, however, 
under the head of curvature ametropia (astigmatism), that 
even- 6 D. cylinder represents about 1 mm. in length, as 
measured on the radius of curvature of the cornea. The 
following table, from Nettleship, gives the exact equiva- 
lents in millimeters for axial ametropia : 



H 



[ 1 D.= o.3 mm. 


M. . . 


. . 1 D. = o.3 mm. 


2D. = 0.5 " 




2D.= o.5 " 


3 D.= i » 




3D-0.9 " 


5D.= i. 5 « 




5D-=i-3 " 


6D.= 2 " 




6 D.= 1.75 " 


9 D.= 3 « 




9D.= 2.6 " 


I2D. = 4 " 




I2D.= 3 .5 « 
i8D.= 5 



Indirect Method. — See page 99 for a full description of 
this method. Slowly withdrawing the objective lens, and the 
disc remaining unchanged in size, signifies emmetropia ; if 
the disc grows uniformly smaller, it means H., and if it 
grows uniformly larger, it means M. (See Fig. 142.) This 
is merely a method of diagnosis, and is never used for 
definite measurements. 
11 



CHAPTER V. 

ASTIGMATISM, OR CURVATURE AMETRO- 
PIA.— TESTS FOR ASTIGMATISM. 

Astigmatism (from the Greek, d, priv. ; ariyna, "a. 
point"). — Optically, astigmatism may be defined as the re- 
fractive condition in which rays of light from a point, passing 
through a lens or series of lenses, do not focus at a point* 

In ophthalmology astigmatism is recognized as that con- 
dition of the refractive system of an eye in which rays of 
light are not refracted equally in all meridians, and the 
resulting image of a point becomes an oval, a line, or a 
circle. (See Fig. ioo.) 

Or astigmatism is that condition of an eye in which there 
are two principal meridians, of greatest and least ametropia, 
each having a different focus. 

In the standard eye the cornea is represented as a section 
of a sphere ; anatomically, however, the cornea is generally 
found to be an ellipsoid of revolution, with its shortest radius 
of curvature (normally y.S mm.) in the vertical meridian. 

In the study of astigmatism the meridians of minimum 
and maximum refraction alone are considered ; they are 
spoken of as the principal meridians, and are at right 
angles to each other. 

With very few exceptions most eyes have some degree 
of astigmatism. The standard or emmetropic eye is an 

*In an article published by Dr. Swan M. Burnett, in "The American 
Journal of Ophthalmology," for December, 1903, entitled "Astigmia or Astig- 
matism," he draws attention to the fact that astigmatism is an erroneous word, 
and gives the true origin of the word from arLyixrj-rjq, meaning a mathematic 
point, whereas "any^a" really means a "blemish" or "brand." He there- 
fore urges the change from astigmatism to astigmia, with the word "astigmic" 
as the adjective. 

122 



ASTIGMATISM. 



123 



extremely rare condition, and plain myopic eyes (long eyes) 
without any astigmatism are almost as rare as the emme- 
tropic condition ; and while plain hyperopic eyes are seen, 
yet statistics show that fully eighty per cent, of hyperopic 
eyes have astigmatism. 

Astigmatism is located in the cornea or lens, or it may 
be a condition of both structures in one and the same eye. 
Astigmatism of the lens may increase, diminish, or neutral- 
ize the corneal astigmatism. Astigmatism, however, is more 
often a condition of the cornea than of the lens. 

Figure 100 shows parallel rays of 
light passing through an astigmatic lens ^y' 

in which the vertical meridian has the 

H' 

V 




Fig. 100. 



shortest radius of curvature, with the result that those rays 
which pass through the vertical meridian V V 7 come to a 
focus before those in the horizontal meridian H H r , which 
has the longest radius. 

Intercepting the refracted rays at I, 2, 3, 4, 5, and 6, the 
image would be at 1 a horizontal oval, at 2 a horizontal 
line, at 3 a circle, at 4 a vertical oval, at 5 a vertical line, 
and at 6 a vertical oval. The space between the points of 
foci of the two meridians (2 and 5) is known as Sturm's 
interval. The importance of this space or interval is that 



124 REFRACTION AND HOW TO REFRACT. 

it represents astigmatism. Sturm's interval is the quantity 
which must be found in correcting astigmatism. 

Causes of Astigmatism. — Most cases of astigmatism 
are congenital, and some can be traced to heredity. Ac- 
quired astigmatism may result from conic cornea, cicatrices 
following ulcers or wounds of the cornea, or be a tempo- 
rary condition from pressure of a chalazion or other 
growth ; and, in fact, astigmatism may develop from any 
disease or injury that will cause a lengthening or shortening 
or inequality in one or more of the meridians of the cornea 
or lens. Swelling of the different sectors of the lens will 
cause astigmatism. The visual line not passing through 
the center of the cornea is a cause of astigmatism, and 
astigmatism is the usual result following extraction of the 
lens. Tenotomy of one or more of the extraocular mus- 
cles will often change the corneal curvature. 

Irregular Lenticular Astigmatism. — This is a normal 
condition of all clear lenses. It is often infinitesimal in 
amount, and on this account does not interfere with vision. 
It is caused by the different sectors of the lens or by the 
individual lens-fibers themselves not being uniform in their 
refracting power. In this form of astigmatism a light does 
not appear to have a distinct edge, but, on the contrary, 
the edge has radiations passing from it, giving the light a 
stellate appearance. There is no known glass that will 
correct this variety of astigmatism. 

Physiologic Astigmatism. — This is due to lid pressure, 
or temporarily to extreme pulling or contraction of the extra- 
ocular muscles. It is a voluntary astigmatism, and therefore 
not constant. It is not a condition of all eyes. The writer 
has demonstrated with the retinoscope and ophthalmometer 
that the condition can be produced in eyes not otherwise 
astigmatic. Drawing the lids together in the act of squint- 



ASTIGMATISM. 125 

ing or frowning, the patient can press the cornea from 
above and below, and give the horizontal meridian of the 
cornea a longer radius of curvature and the vertical meri- 
dian a shorter radius ; or with the eye looking into the 
telescope of the ophthalmometer, no overlapping of the 
mires is noted, but in some instances when told to open the 
eye widely and "stare" into the instrument, as much as 
yi or 3/( of a diopter of astigmatism may be recorded. 

This "transient" astigmatism should never be corrected 
with a glass. 

Subdivisions of Astigmatism. — In addition to the 
astigmatisms just described, curvature ametropia has been 
further considered as : 



I 


Irregular. 






6. 


Astigmatism against the 


2 


Regular. 








rule. 


3 


Symmetric. 






7- 


Homonymous. 


4 


Asymmetric. 






8. 


Heteronymous. 


5- 


Astigmatism 


with 


the 


9- 


Homologous. 




rule. 






10. 


Heterologous. 



i. Irregular Astigmatism. — This is usually located in 
the cornea, and is due primarily to some breach in the 
continuity of one or more of its meridians ; for example, 
the vertical meridian may appear regular, but the hori- 
zontal meridian is not a uniform curve, but is irregular at 
some point or points. Such meridians can not produce 
clear retinal images, but, on the contrary, the resulting 
retinal image is hazy or irregular. 

2. Regular Astigmatism. — In this variety the cornea 
and lens are regular in their curvatures, from the maximum 
to the minimum radius, and the retinal image can be made 
clear with correcting glasses. 

Before entering upon the study of the various forms of 
regular astigmatism, the student's attention is called to two 



126 REFRACTION AND HOW TO REFRACT. 

important facts : (a) That, as a rule, the shortest radius of 
curvature of the cornea is in the vertical meridian — that is 
to say, the vertical meridian has a stronger refracting power 
than the horizontal. 

(p) The student should bear in mind that in the measure- 
ment of curvature ametropia each millimeter of lengthening 
or shortening of the radius of curvature is equivalent to a 
6 D. cylinder. For instance, an eye which requires a + 6 D. 
cylinder axis 90 degrees has the horizontal radius of curva- 
ture about one millimeter longer than the vertical radius ; 
or an eye that requires a — 6 D. cylinder axis 1 80 degrees 
has its vertical radius of curvature about one millimeter 
shorter than the horizontal. In axial ametropia, however, 
it was shown that every three diopter sphere represented 
about one millimeter in length, as measured on the axis. 

Varieties of Regular Astigmatism. — There are five 
different forms of regular astigmatism : 
(a) Simple hyperopic. (V) Compound hyperopic. 

(&) Simple myopic. (a 7 ) Compound myopic. 

(e) Mixed astigmatism. 
(a) Simple Hyperopic Astigmatism. — Abbreviated As. 
H., or H. As., or Ah. About $*4 per cent, of eyes have 

this form of refraction. This is 

X""^ ^"N. a condition where one meridian 

h^^2jT^-\ \ °^ ^ e e y e * s emme tropic, and 

Qj ~~~~_I^^'4--— h ' tne meridian at right angles to 

__Z_SZ— —^^ J it is hyperopic (see Fig. 10 1); 

V N. / the vertical meridian focuses 

FlG IOI parallel rays on the retina, and 

the horizontal meridian would 

focus back of it. The retinal image of a point is a line, 

usually horizontal. (See 2, in Fig. 100.) The correcting 

lens is a plus cylinder with its axis usually at 90 degrees, 



ASTIGMATISM. 



127 




Fig. 102. 



or within 45 degrees of 90 degrees. Example, +2.00 
cylinder axis 90 degrees. 

(6) Simple Myopic Astigmatism. — Abbreviated As. M., 
or M. As., or Am. This is not a common condition. About 
I y 2 per cent, of all eyes have this form of astigmatism. 
This is a condition where one meridian of the eye is emme- 
iropic, and the meridian at right angles to it is myopic (see 
Fig. 102); the horizontal meridian 
focuses parallel rays on the retina, 
and the vertical meridian focuses 
parallel rays in front of the retina 
(in the vitreous), with the result 
that they cross before reaching 
the retina. The retinal image of 
a point is a line, usually vertical. 

(See Fig. 100.) The correcting lens is a minus cylinder with 
its axis at 180 degrees, or within 45 degrees of 180 degrees. 
Example, — 2.50 cylinder axis 180 degrees. 

(c) Compound Hyperopic Astigmatism. — Abbreviated 
H. As. Co., or Comp. Has., or H-f Ah (hyperopia com- 
bined with astigmatism hyperopic). This condition repre- 
sents nearly forty-four per cent, of all eyes ; it is the most 

common of all forms of re- 
fraction. 

The retinal image of a 
point is an oval ; never a line 
and never a circle. (See 1, 
in Fig. 100.) 

The correcting lenses are a 
plus sphere and a plus cylin- 
der. Example, 4-2.00 S. O +3-00 cylinder axis 90 de- 
grees. Compound hyperopic astigmatism is a combination 
of axial ametropia (short eye) and simple hyperopic astig- 




128 REFRACTION AND HOW TO REFRACT. 

matism (curvature ametropia). In this form of astigmatism 
both meridians have their foci back of the retina — one 
further back than the other. The retina intercepts the rays 
before they can focus. Figure 103 shows this condition. 
Usually the vertical meridian focuses nearer the retina than 
the horizontal. 

(d) Compound Myopic Astigmatism. — Abbreviated M. 
As. Co., or Comp. Mas., or M. + Am. (myopia combined 
with astigmatism myopic). This is by far the most com- 
mon condition of all myopic eyes, and represents about 
eight per cent, of all eyes. 

The retinal image of a point is always an oval ; never a 
line or a circle. (See 6, in Fig. 
100.) 

The correcting lenses are a 
minus sphere and a minus cylin- 
der. Example, — 1 sph. O — 2 
cylinder axis 180 degrees. A 
combination of axial ametropia 
Fig. 104. (long e y e ) an d simple myopic 

astigmatism. 
Figure 104 shows that parallel rays have two points of 
foci in front of the retina — one further front than the 
other. 

(e) Mixed Astigmatism. — This form of refraction is 
found in about 6*^ per cent, of all eyes, and is abbreviated 
in three different ways : 

1. Ah + Am. (astigmatism hyperopic with astigmatism 
myopic). 

2. H+Am. (hyperopia with astigmatism myopic). 

3. M-f-Ah. (myopia with astigmatism hyperopic). 

The retinal image of a point is an oval or a circle ; never 
a line. (See 3 and 4 in Fig. 100.) 




ASTIGMATISM. 



129 



The correcting lenses are one of three combinations, and 
spoken of as crossed cylinders. Examples : 

1. — 1. 00 cyl. axis 90 degrees 3 — 2 -°° c )^- ax i s l %° degrees. 

2. 4-1 S. 3 — 3 c yl- axis 180 degrees (cylinder always stronger than the 
sphere). 

3. — 2 S. 3 ~r3 c yl*- ax i § 9° degrees (cylinder always stronger than the 
sphere) . 

The condition of mixed astigmatism is one of simple 
hyperopic astigmatism, with simple myopic astigmatism : 
one meridian focuses parallel rays in front of the retina and 
the other meridian (at right angles) focuses parallel rays 





Fig. 105. 



Fig. 106. 



back of the retina. Figures 105 and 106 show this arrange- 
ment. 

The remaining subdivisions of astigmatism are merely 
classifications of the different forms already described, and 
arise from a study of the axis of shortest radius of curva- 
ture. 

3. Symmetric Astigmatism. — When the combined 
values, in degrees, of the meridians of shortest or longest 
radii of curvature in both eyes equal 180 degrees (no more 
and no less), then the astigmatism in the two eyes is spoken 
of as symmetric. For example, if the cylinder in the right 
eye is at axis 75 degrees, and in the left eye at 105 de- 
grees ; 75 degrees and 105 degrees added together will 



130 



REFRACTION AND HOW TO REFRACT. 



make 180 degrees. (See Fig. 107.) Or if each eye takes 
a cylinder axis at 90 degrees, they are also symmetric, 90 
degrees and 90 degrees making 1 80 degrees. If both eyes 
have axes 180 degrees, they are symmetric also, one 
meridian being considered as zero (o). 

4. Asymmetric astigmatism is the reverse of sym- 
metric, and is, therefore, the condition in which the com- 
bined values, in degrees, of the cylinder axes do not make 
180 degrees. For instance, if the right eye has a cylinder 
at axis 75 degrees and the left at 120 degrees, these 




80 



75° 90° 105° 



Fig. 107. — Illustrating Symmetric 
Astigmatism. 




75 90 



Fig. ic8. — Illustrating Asymmetric 
Astigmatism. 



added together would not make 180 degrees, but more than 
180 degrees. (See Fig. 108.) Or, if the astigmatism in 
the right eye was at 35 degrees, and the left at 90 degrees, 
these added together would not make 180 degrees. 

Symmetric astigmatism generally accompanies a regular 
physiognomy, the center of each pupil being at an equal 
distance from the median line of the face. Asymmetric 
astigmatism usually accompanies an asymmetric physiog- 
nomy, the center of one pupil being further from the 
median line of the face than the other. 



ASTIGMATISM. 



131 



Muscular insufficiency, hereafter to be described, is much 
more common, and, in fact, should be looked for or antici- 
pated in cases of asymmetric astigmatism. 

5 and 6. Astigmatism with the Rule and Astigmatism 
Against the Rule. — Astigmatism with the rule and astig- 
matism against the rule refer to the condition already 
described as that in which the vertical meridian of the eye, 
as a general rule, has the shortest radius of curvature. 

Statistic tables on astigmatism show that most eyes 
accept a plus cylinder at axis 90 degrees, or within 45 




80° 




180° 



Fig. 109. — Illustrating Astigmatism 
with the Rule. 



Fig. 



no. — Illustrating Astigmatism 
against the Rule. 



degrees (inclusive) either side of 90 degrees (see Fig. 
109); or a minus cylinder at axis 180 degrees, or within 
45 degrees (inclusive) either side of 180 degrees. For 
example, if an eye requires a plus cylinder at 45 degrees, 
or at any axis from 45 degrees up to 135 degrees (inclu- 
sive), taking axis 90 as the median line, then the astig- 
matism is with the rule. But if an eye should require a 
plus cylinder within 45 degrees either side of 180 degrees, 
then the condition is one of astigmatism against the rule. 
(See Fig. 1 10.) A plus or minus cylinder at 45 degrees 



132 REFRACTION AND HOW TO REFRACT. 

or 135 degrees is recognized as astigmatism with the 
rule. 

7. Homonymous astigmatism is the condition in which 
the cylinder axis in each eye is the same. 

8. Heteronymous astigmatism is the condition in which 
the astigmatism in one eye is with the rule, and in the other 
eye against the rule. For example : 

O. D. -f-2 cyl. axis 90 degrees, and O. S. 4 2.00 cyl. axis 180 degrees. 

9. Homologous astigmatism is symmetric astigmatism 
with the rule — i e. : 

O. D. -f I -°° c yl- ax i s 60 degrees, O. S. -j-i.oo cyl. axis 120 degrees. 

10. Heterologous astigmatism is symmetric astigma- 
tism against the rule — i. e. : 

O. D. -f- 1 - 00 cyl. axis 15 degrees, O. S. 4- 1 - 00 cyl. axis 165 degrees. 

Meridians of the Eye. — The various axes or meridians 
of the eye are indicated by degree markings on the peri- 
phery of the trial-frame, and by corresponding imaginary 
lines drawn around the eyeball from the anterior pole or 
apex of the cornea to the posterior pole. 

Either eye (right or left) is exactly like its fellow, and is 
numbered by starting from zero (o) on the left-hand side of 
the horizontal meridian and counting downward to the right- 
hand side until this same line is again reached. This makes 
half a circle (hemicircle) of 180 degrees. (See Fig. 109.) 
As the degrees in this half-circle are all carried across the 
eye, they maintain their individual numbering, so that axes 
5, 10, 15, etc., are the same whether above or below the 
horizontal meridian. Hence there is no reason for having 
a complete circle of 360 degrees. Some trial -frames have 
the upper, while others have the lower, half numbered ; 
this makes no difference in the exact numbering ; in one in- 
stance the count is made from left to right, and in the other 



ASTIGMATISM. 1 33 

the count is made from right to left. The foreign trial-frame, 
as represented on page 48, may be confusing if not studied. 

Symptoms of Astigmatism. — More aggravated symp- 
toms of accommodative asthenopia are apt to be detailed 
by the patient, but there are, in truth, no definite symptoms 
whereby the presence of astigmatism can be positively dif- 
ferentiated from axial ametropia. The diagnosis of astig- 
matism by the physiognomy is confirmed only because 
most eyes are astigmatic ; the simple hyperopic eye squints 
the eyelids together just the same as the eye that is astig- 
matic, so that the writer would not diagnose astigmatism 
by the patient's individual history of his eyes. 

How to Diagnose Astigmatism. — This is one of the 
very early questions of the beginner in ophthalmology. 
Astigmatism being the prominent factor in almost all refrac- 
tive work, the writer feels justified in giving this part of 
refraction extensive explanation. Of the various methods 
of diagnosing astigmatism the writer would mention the 
following : 

1. Corneal reflex. io. Chromo-aberration or cobalt-blue 

2. Confusion letters. glass test. 

3. Placido's disc. II. Thomson's ametrometer. 

4. Stenopeic slit. 12. The ophthalmometer. 

5. Astigmatic chart. 13. Direct ophthalmoscopy 

6. The pointed line test. 14. Indirect ophthalmoscopy. 

7. Perforated chart or disc. 15. Cylindric lenses. 

8. Pray's letters. 16. Retinoscope. 

9. Scheiner's Test. 

I. The Corneal Reflex Test. — The cornea and under- 
lying aqueous representing a spheric mirror, naturally fur- 
nish a small image of surrounding objects. If the cornea 
is astigmatic, the catoptric image must be correspondingly 
distorted. To make the examination, the patient stands 
facing a window, and the surgeon at one side observes the 



134 REFRACTION AND HOW TO REFRACT. 

image of the window-panes in the corneal mirror ; these 
will be broadened or lengthened, or they may appear in- 
clined to one side, according to the axis and character of 
the astigmatism. This test is not commonly used, is often 
overlooked ; in fact, unless the astigmatism is of consid- 
erable degree, is not a valuable test. 

2. Confusion Letters. — Letters on the card which is 
used for testing distant vision are arranged in such order 
that those which have a resemblance are placed next to 
each other. (Fig. 74.) For example, X and K, Z and E, 
O and D, C and G, P and F, S and B, V and Y, H 
and N, A and R, etc. The patient, in deciphering these 
letters in the line corresponding to his best vision, often 
miscalls them, and can not tell an X from a K, or a Z 
from an E, etc. These letters are, therefore, spoken of as 
confusion letters. This is a very good general test, but is 
not infallible, as a patient with opacities- in the media will 
make similar mistakes. 

3. Placido's Disc or Keratometer (see Fig. 1 1 1). — To a 
wooden handle is secured a round piece of thin sheet-iron 
eight inches in diameter, and at its center is a small, round 
5 mm. opening. On one side the disc is painted in alternate 
concentric circles or bands in black and white; these circles 
are not equidistant, the radii of the several circles being cal- 
culated according to the law of tangents, so that when re- 
flected on a cornea of spheric curvature they appear equi- 
distant in the image. On the reverse side is placed a slot to 
hold a convex lens for magnifying purposes. To use this 
disc, the patient is placed with bis back to a strong light from 
a window, or an artificial light may be placed over his head. 
The surgeon holds the disc with the sight-hole close in front 
of his own eye, and with the light illuminating the disc, the 
patient is instructed to look into the perforation. The sur- 



ASTIGMATISM. 



135 



geon then approaches the eye until the corneal image of 
the outer edge of the instrument corresponds to the outer 
edge of the patient's cornea. When this distance is 
reached, a convex 2, 3, or 4 D. sphere may be placed in 
the slot of the disc so as to magnify the corneal image. 
If the cornea is not astigmatic, then the black and white 

circles will appear uni- 
form throughout ; but if 
there is astigmatism, the 
circles will appear more 
or less oval. If irregu- 
lar astigmatism or conic 
cornea is present, the cir- 
cles will appear broken 
or distorted in certain 
parts. This test has be- 
come almost obsolete. 





Fig. hi. 



Fig. 112. 



4. Stenopeic Slit (see Fig. 1 12). — This is a round metal 
disc of the size of the trial-lens, and contains a central slit 
or opening about 25 mm. long and 1 or 2 mm. wide. The 
stenopeic slits sold in the shops have various breadths of 
openings, from ]/ 2 to 2 mm. ; that with the 1 mm. opening 



I36 REFRACTION AND HOW TO REFRACT. 

is the one recommended. The purpose of the slit is to 
cut off or exclude all rays of light at right angles to its 
position in front of the eye. When placed at axis 90, all 
rays in the horizontal meridian are excluded ; when placed 
at axis 1 80, all rays in the vertical meridian are cut off, etc. 
To use the stenopeic slit, place it in the trial -frame in front 
of the eye to be examined, the fellow-eye being covered. 
The patient is instructed to read the letters on the distant 
test-card, and as he does so, the slit is slowly turned 
through the different meridians. If the vision remains the 
same, no matter through which meridian the patient reads, 
astigmatism may be absent ; but if the patient selects one 
meridian in which he sees best, and another meridian at 
right angles in which he does not see so well, astigmatism is 
usually present. For instance, if the slit is at axis 75 degrees 
and the patient reads ^J, and at axis 165 he reads ^, then 
he is astigmatic in the 165 meridian. The amount of the 
astigmatism can be calculated by placing spheric lenses 
back of the slit and finding the difference in strength of 
the spheres which bring the vision up to the normal. For 
example, when the slit is at axis 75 and the patient reads 
J^, if a -j- 1.50 S. is used, and the vision becomes ^j, then 
1.50 corrects axis 75. Turning the slit to axis 165, and 
proceeding in the same way, if +2.50 S. brings the vision 
from ^ to y}, then +2.50 corrects axis 165, the differ- 
ence between the +1.50 and +2.50 being 1 D., and the 
formula would be -f-1.50 sph. <^ -j-i.oo cyl. axis 75 
degrees. This test is not often used, and when resorted 
to, the eyes should be under the influence of a cyclo- 
plegic. This test is of special service in some cases of 
mixed astigmatism, irregular astigmatism, presbyopia, and 
aphakia. 



ASTIGMATISM. 



137 



5. Astigmatic Chart. — There is an infinite variety of 
these cards (see Fig. 113), and the student is puzzled 




FlG. 113. — Astigmatic Charts of Dr. John (Ji 



which one to select. Ordinarily, the " clock-dial " will 
answer every purpose. (Fig. 114.) This is a white 



I38 REFRACTION AND HOW TO REFRACT. 

card * with peripheral Roman characters corresponding to 
the characters on the clock-face, hence its name. From 
these figures a series of three parallel and uniformly black 
lines, with interspaces of the same width as the lines, cross 
from XII to VI, III to IX, IIII to X, V to XI, VII to I, 
and VIII to II. This chart should be so calculated that 




Fig. 114. 

the lines and interspaces will form an angle of 5 minutes in 
width consistent with the distance at which the test is to be 
made : if at six meters, 8.7 mm. ; if at four meters, 5.7 mm. 
In most charts the lines subtend an angle much greater 
than 5 minutes for the distance at which they are used, and 
in this way the true delicacy of the test for small errors or 

* A black card with white lines is also used. (See Fig. 115.) 



ASTIGMATISM. 



139 



amounts of astigmatism is sacrificed. The purpose of the 
chart is to detect, by the patient's answer, whether astigma- 
tism is present, and, if so, in which meridian. 

The chart, illuminated by reflection from a steady artificial 
light, is placed on a horizontal line perpendicular to the 
patient's eyes ; it should never be hung at an angle, and 
must always be perfectly flat. Each eye is to be tested 
separately. Looking at such a chart, if all the lines appear 




equally black, astigmatism of any considerable degree or 
amount may often be excluded ; but if the patient selects 
one series of lines as darker than others, then the presence 
of astigmatism may be diagnosed. If the astigmatism is 
of a very high degree, the patient may see the three lines 
as one solid black line without interspaces. 

Rule i. — The meridian of the eye which corresponds to 
the dark lines selected is the meridian of astigmatism. 

Example. — If the horizontal lines (from III to IX) appear 



I4O REFRACTION AND HOW TO REFRACT. 

darker than all the others, then it is the horizontal meridian 
(o or 180 degrees) of the eye which is astigmatic. Or if 
the lines from VI to XII are darkest, then the vertical mer- 
idian of the eye is astigmatic. In other words, the series 
of darkest lines indicates the meridian of greatest ametropia. 

Rule 2. — The axis of the cylinder in the prescription 
will be opposite to the meridian of the dark lines. 

Example. — A patient who requires a plus cylinder at axis 
90 degrees sees the horizontal lines (from III to IX) as very 
dark, and the lines from VI to XII not so dark, and the axis 
of the cylinder in the prescription will be opposite to 1 80 
degrees — i. e. } at 90 degrees. 

According to the definition of "astigmatism with the 
rule " and " astigmatism against the rule," it follows that, 
with few exceptions, those patients who select a series 
of lines at 180 degrees, or within 45 degrees either side of 
180 degrees, as darker than other lines, have hyperopic 
astigmatism, whereas those who select a series of lines at 90 
degrees, or within 45 degrees either side of 90 degrees, 
have myopic astigmatism. 

According to the definition of symmetric astigmatism, a 
patient's right eye selecting the lines at 90 or 180 as 
darker than those at right angles, will select the same series 
of dark lines in the left eye. If the series of dark lines with 
the right eye is from II to VIII, then the left eye selects 
the dark lines from X to IV, etc. 

The clock-dial is the form of chart in common use, 
and as a test for astigmatism is not without considerable 
merit. 

When the astigmatism is of small amount, it may not be 
recognized by means of the clock-dial until after the spheric 
correction has been placed before the eye or after a cyclo- 
plegic has been instilled. 



ASTIGMATISM. 



I 4 I 



6. The writer's pointed line test, as shown in figure 1 16, 
is a series of one-minute black squares, in three parallel 
lines at right angles to each other, on a cream-colored card ; 
the squares and adjoining spaces making a five-minute 
angle for six meters. By means of a clockwork and bat- 
tery, this dial may be revolved by pressing a button. The 
principle of the test is the same as the perforated disc. 




Fig. 116. 



7. The Perforated Disc (Fig. 117). — This is a modifica- 
tion of the astigmatic chart. A piece of white cardboard 
or metal, about ten inches square, has small, round perfora- 
tions made in it of certain definite size. Each perforation is 
separated from its neighbor by the distance of its diameter. 
These openings are arranged in series of one, two, or three 
parallel lines, exactly as in the pointed line test. This 
chart or disc is hung on the window-pane, or an illumina- 
tion is placed behind it. The patient, looking at the disc, 



142 



REFRACTION AND HOW TO REFRACT. 



signifies which series of perforations appear to coalesce and 
form lines. This test is not commonly known or used. It 




Fig. 117. 



^== ^=^= *§*^ 

m u Jr 

IPW ip\ "/"i 



might be a valuable test if there was any convenient way 

of uniformly illuminating it from behind. 

8. Pray's Letters (Fig. 118). — 
These letters are of the Old English 
type, and composed of strokes which 
run in different meridians. The pa- 
tient, looking at these letters, selects 
that letter which appears darker than 
all the rest. The direction of the lines 
in the letter selected corresponds to the 
meridian of greatest ametropia. This 
test is very confusing to the patient, 
Fig. 118. wno sees fi rst one letter and then an- 

other as darker than its fellows. 
Schemer's Test. — This is an old test for ametropia, 






ni;;.n 



11 



ASTIGMATISM. 



H3 



good in theory, but really not sufficiently accurate for prac- 
tical purposes. It is explained for the student's information, 
and not with the idea that he will ever take time to use it. 
The test is made with a small piece of metal (Fig. 1 19) 
the size of the trial-lens, which contains two pin-point round 
openings at its center, separated by an interval of two or 
three millimeters. One of these openings is covered with a 
red glass, as suggested by Dr. Wm. Thomson. This disc 
is placed close to the eye, so that light may pass through 
both openings into the eye at one and the same time. The 
eye, if not presbyopic, should be under the influence of a 




Fig. 119. 




cycloplegic. The eye looks at a distant point of light. The 
principle of the test depends upon which part of the retina 
is stimulated by the rays entering the eye through these 
openings, all other rays being excluded. The student must 
remember that rays which fall upon the temporal side of the 
retina are referred to the nasal side ; those which fall upon 
the nasal side of the retina are referred to the temporal side ; 
those which fall upon the lower portion of the retina appear 
to come from above ; and those which fall upon the upper 
portion of the retina appear to come from below. 

Diagnosis of Hyperopia ( Fig. 1 20). — The disc is placed 
with the red glass (R) above. The patient then sees a red 



144 



REFRACTION AND HOW TO REFRACT. 




and a white light (W). The red appears below the white. 
Gradually revolving the disc, the two lights move, and keep 
the relative positions and distance apart. The greater the 
distance between the two lights, the higher the refraction 
or amount of the hyperopia. That plus sphere placed in 
front of the disc which unites the two flames into one (pink) 
flame is the approximate amount of the hyperopia. 

Diagnosis of Myopia (Fig. 121). — Placing the disc 

before the eye as before, with 
the red glass (R) above, the 
patient sees the red flame 
above the white (W). Gradu- 
ally revolving the disc, these 
two lights keep their rela- 
tive positions and distance. 
That minus sphere placed 
before the disc which makes 
the two lights appear as one (pink) light is the approximate 
amount of the myopia. 

Diagnosis of Emmetropia. — This condition would give 
but one light (pink in color), and unchanged by rotating 
the disc. 

Diagnosis of Simple Hyperopic Astigmatism. — One 
meridian appears the same as in emmetropia, and the meri- 
dian opposite to the emmetropic meridian would show a 
separation of the two lights, as in hyperopia. The plus cyl- 
inder placed before the disc which unites the two lights in the 
ametropic meridian represents the amount of the astigma- 
tism. 

Diagnosis of Simple Myopic Astigmatism. — The 
lights are red and white in one meridian, as in simple hyper- 
opic astigmatism, but the red light is seen in the direction 
of the red glass, and when the disc is rotated to the oppo- 



ASTIGMATISM. 



145 



site meridian, only one light appears, and of a pink color. 
The amount of the astigmatism is represented by the 
strength of minus cylinder which brings the two lights 
together in the ametropic meridian. 

Diagnosis of Compound Hyperopic Astigmatism. — All 
meridians show two lights, the red light being in the direc- 
tion of the clear opening in the disc, but one meridian will 
show a greater separation of the lights than in the meridian 
at right angles. To find the correction and the amount of 
the astigmatism, proceed as in simple hyperopia, correcting 
each meridian separately with a sphere. 




Fig. 122. 



Fig. 123. 



Diagnosis of Compound Myopic Astigmatism. — This 
is the same as in compound hyperopic astigmatism, with a 
reversal of the position of the lights, and the amount of the 
ametropia is obtained with minus spheres. 

Diagnosis of Mixed Astigmatism. — One meridian 
appears as in simple hyperopic astigmatism, and the meri- 
dian opposite to it appears as in simple myopic astigma- 
tism. The amount of the astigmatism is calculated as in 
these two conditions. 

10. Chromo-aberration Test. — This is also known 
as the cobalt-blue glass test. Cobalt is a mineral, and is 
13 



I46 REFRACTION AND HOW TO REFRACT. 

used as a coloring-matter by glass-blowers. Cobalt-blue 
glass comes in two forms : one in which the glass is colored 
throughout, and the other in which it is colored on only one 
surface, known as " flashed." To the eye, cobalt-blue glass 
appears dark blue, but contains a great deal of red. For 
purposes of testing ametropia, a dark shade of blue should 
be selected, or two or three pieces of a light shade may be 
cemented together so as to give the desired dark shade. 
This glass is cut round and fitted into a trial-cell. (See 
Figs. 122 and 123.) 

The power of cobalt-blue glass to exclude all but blue 



Fig. 124. 

and red rays gives this test its principle. Blue rays being 
more refrangible than red, naturally focus sooner than red. 
Red rays will focus back of the blue. (See Fig. 124.) 

There are several important details in the use of this 
test which must be carefully executed if definite results are 
to be obtained : 

1 . The eye should be under the influence of a cycloplegic. 

2. By means of a light-screen, a small round area of 
steady white light should be looked at from a distance of 
four or six meters. 

3. Each eye is to be tested separately. 



ASTIGMATISM. 



147 






Fig. 125 




Fig. 127. 




Fig. 128. 



Fig. 129 



Fig. 130. 





Fig. i ;i. 



Fig. 132. 



Fig. 133. 



Fig. 134. 



Fig. 135. 



Fig. 136. 



125. High hyperopia. 126. High myopia. 127. Low simple hyperopic as- 
tigmatism. 128. High simple hyperopic astigmatism. 129. Low simple 
myopic astigmatism. 130. High simple myopic astigmatism. 131. Low 
compound hyperopic astigmatism. 132. High compound hyperopic astig- 
matism. 133. Low compound myopic astigmatism. 134. High com- 
pound myopic astigmatism. 135. Low mixed astigmatism. 136. High 
mixed astigmatism. 



I48 REFRACTION AND HOW TO REFRACT. 

4. The cobalt glass may be placed near the flame or, 
better still, close in front of the patient's eye ; in every 
instance it must be perpendicular to the front of the eye, 
and never at an angle. 

5. All other lights except the one in use should be 
excluded. 

Diagnosis of Emmetropia. — Patient sees a small circle 
composed of two colors equally mixed ; purple. (See E 
in Fig. 124.) 

Diagnosis of Hyperopia (see H in Fig. 124). — The 
patient describes a red ring of light with a blue center. 

Diagnosis of Myopia. — The patient describes a blue 
ring with a red center. (See M in Fig. 124.) 

Diagnosis of Astigmatism. — If astigmatic, then he will 
describe one of the conditions as shown on page 147. If 
the test is made as suggested, it will have three points of 
recommendation : 

1. The character of the refraction is quickly diagnosed. 

2. It may lead to an early diagnosis of red-blindness, a 
condition often overlooked. 

3. Likewise it will show a central scotoma for red in 
advanced toxic amblyopia, if the eye is made myopic with 
a plus sphere. 

11. Thomson's Ametrometer (Fig. 137). — This instru- 
ment has two small gas-flames about five millimeters in diam- 
eter, one stationary and the other movable on a metal arm, 
which can be changed or revolved to any meridian. Each 
eye is tested separately at a distance of twenty feet, and 
preferably under a cycloplegic. The method of the test is 
to move one flame along the metal arm until the two lights 
appear to fuse. The scale, as marked on the arm, gives 
the approximate strength of lens necessary to correct the 
ametropia. By raising or lowering the arm any meridian 



ASTIGMATISM. 



149 



may be tested. It is a most ingenious test, but not in 
common use. 

12. The Ophthalmometer (see Figs. 138 and 139). — 
This name literally means an " eye measure," but as the in- 
strument measures only the different radii of corneal curva- 
ture, a much better name would be keratometer, or measure 
of the corneal radii. The object of the ophthalmometer is 




Fig. 137. 



the measurement of corneal curves by means of catoptric 
images viewed through a telescope. 

The ophthalmometer consists of a telescope which con- 
tains a Wollaston birefrangent prism placed between two bi- 
convex lenses. Attached to the telescope is a graduated arc, 
upon which are placed two white enameled objects called 
mires (targets). (See Figs. 139, 140, 141.) The left mire 
is stationary, and is made up of two 3 cm. squares, separated 



150 



REFRACTION AND HOW TO REFRACT. 



by a black line 2 mm. wide ; the right mire is movable and 
graduated into steps, each 5 mm. wide ; a black line passes 
through the middle of these steps. For purposes of focus- 
ing, the telescope is mounted on a movable tripod. The 
patient is seated with his chin and forehead resting in a 




Fig. 138. 



frame. At the side of the frame, and attached to it, are two 
or four electric lights or Argand burners, which illuminate 
the mires. The surgeon, looking through the eye -piece of 
the telescope, focuses the center of the patient's cornea 
until he sees two images of each mire clearly ; then he 
selects the two central images for further study and ignores 



ASTIGMATISM. I 5 I 

the peripheral images. The next step is to move the 
right-hand mire until these two images of the mires occupy 
the center or pole of the cornea, so that their inner edges 
just touch and the black line in each makes one continu- 
ous black line through both (see Fig. 140) ; and to do the 




Fig. 139. 

latter, the barrel of the telescope may have to be gradually 
revolved from left to right or right to left, but never more 
than 45 degrees either way. When this position is ob- 
tained, the axis or meridian is noted by the arrow, which 
points to the figure on the dial at the back of the arc, or, 
as in some old instruments, on the front of the dial. This 



152 REFRACTION AND HOW TO REFRACT. 

position of the mires is spoken of as the primary posi- 
tion. 

Revolving the telescope to the opposite meridian (mer- 
idian at right angles), which is called the secondary posi- 
tion, the observer notes any change which may have taken 
place in the relative positions of the mires. If they have 
not changed, but still maintain their edges in apposition, as 
in the primary position, then the cornea has a uniform cur- 
vature thoughout, and there is no astigmatism of the 
cornea present. If, however, when the secondary position 
is reached and the catoptric image of the mires with the 
steps has encroached upon the catoptric image of the sta- 
tionary mire, then the astigmatism is calculated by the 

amount of this overlapping. 
(See Fig. 141.) 

Each step representing one 
diopter of astigmatism, one- 
half a step of overlapping 
FlG I40 FlG# I4I would represent half a diopter, 

etc. If, in making the change 
from the primary to the secondary position, the mires 
should separate, then the surgeon would know that his 
secondary position should have been his primary position, 
and he will have to make a corresponding change. 

As already stated, lenticular astigmatism is not a condi- 
tion to be ignored, as only too often it will increase, 
diminish, or even neutralize corneal astigmatism, so that in 
point of fact the ophthalmometric findings are more often 
useless than of real value in estimating the total refractive 
error. Cylinders should never be prescribed from the 
ophthalmometric findings until carefully confirmed by 
other and much more reliable tests. As a keratometer, 
the instrument can not be excelled, and, therefore, it has a 




ASTIGMATISM. I 5 3 

place in testing the refraction in cases of aphakia. The 
ophthalmometer as a means of diagnosis is suggestive 
rather than positive. 

13. Estimation of Curvature Ametropia (Astigma- 
tism) with the Ophthalmoscope, Direct Method. — The 
presence of astigmatism is diagnosed by the direct method 
from the fact that the vessels or details of the fundus are 
not all seen clearly with one and the same glass in the 
ophthalmoscope ; in other words, the vessels passing up 
and down on the disc are seen clearly with a different lens 
in the ophthalmoscope than is required to see the vessels 
passing laterally or at right angles. The amount of the 
astigmatism is the difference in the strength of the respective 
lenses used for this purpose ; for instance, if the vertical 
vessels are seen best with a -j-4 S., and the horizontal 
vessels with a -f-2 S., then the amount of the astigmatism 
would be -\-2 D. 

When using the ophthalmoscope for making refractive 
estimates in astigmatic eyes, the student should remember 
that the glass with which a vessel is seen distinctly in one 
meridian represents the amount of the refraction in the 
meridian at right angles to this vessel. In other words, 
each vessel in the eye-ground of an astigmatic eye is seen 
clearest through the meridian at right angles to its course. 
This is a puzzle to the beginner, but he must remember that 
cylinders refract opposite to their axes. In estimating the 
refraction with the ophthalmoscope, the observer looks first 
at the shape of the disc ; if it appears oval, this would be an 
evidence of astigmatism ; secondly, if the upper and lower 
edges of the disc are seen clearly with a different strength 
glass than that required to see the inner and outer margins, 
then this would be a further evidence of the presence of 
astigmatism ; but the third and confirmatory test of the 
presence of astigmatism should be the different strength 



154 REFRACTION AND HOW TO REFRACT. 

glasses required to see the vessels distinctly in the neigh- 
borhood of the macula. An eye having an oval nerve, 
whose edges can all be seen clearly with one and the same 
glass in the ophthalmoscope is not usually astigmatic. 

Examples of estimated refraction by the direct method. 

Simple Hyperopic Astigmatism. — Vertical vessels seen 
with a + 1 S. and horizontal vessels seen without any lens 
would equal +1.00 cyl. axis 90 degrees. 

Simple Myopic Astigmatism. — Vertical vessels seen 
without any lens and horizontal vessels seen with — 3 S. 
would equal — 3 cyl. axis 180 degrees. 

Compound Hyperopic Astigmatism. — Vertical vessels 
seen with +4 S. and horizontal vessels seen with -j-3 S. 
would equal -f-3.00 S. O + 1. OO cyl. axis 90 degrees. 

Compound Myopic Astigmatism. — Vertical vessels 
seen with — 2 S. and horizontal vessels seen with — 5 S. 
would equal — 2.00 S. O — 3.00 cyl. axis 180 degrees. 

Mixed Astigmatism. — Vertical vessels seen with 
+ 2 S. and horizontal vessels seen with — 3 S. would equal 
— 3.00 S. O +5-00 cyl. axis 90 degrees. 

14. Diagnosis of the Character of the Refraction by 
the Indirect Method (see Fig. 142 and p. 99). — There is 
nothing exact about this method, and the refractive error, 
to be recognized, must be considerable. 

1. Gradually withdrawing the lens (objective) from in 
front of the eye, if the aerial image of the disc retains its 
uniform size in one meridian, it signifies emmetropia for that 
meridian ; but if it grows smaller in one meridian, that 
meridian is hyperopic ; or if larger, then that meridian is 
myopic. 

2. If the image grows smaller, but more so in one meridian 
than the other, it signifies compound hyperopia. If the 
image grows larger, but more so in one meridian than the 
other, then the condition is one of compound myopia. The 



ASTIGMATISM. 



155 



image growing smaller in one meridian, while in the other 
it grows larger, indicates mixed astigmatism. 




Fig. 142. — Companion picture to figure 89. Illustrating the indirect method. 
Rays from the lamp (L) are reflected convergently from the mirror of the 
ophthalmoscope, and, passing through the convex lens and into the eye, 
produce a large retinal illumination, extending from I to I. T B are rays 
from the edge of the disc, and, leaving the eye parallel, pass through the 
convex lens and form an inverted aerial image of the disc at T / B'. The 
-j-4 S. in the ophthalmoscope magnifies the image T / B'. 



15. The cylinder lens test for astigmatism is described 
under Applied Refraction, page 251. 

16. Retinoscopy is described in chapter VI. 



CHAPTER VI. 
RETINOSCOPY. 
Retinoscopy, or the Shadow Test.— This may be de 




Fig. 143. — The Author's Schematic Eye for Studying Retinoscopy. 

fined as the method of estimating the refraction of an eye 
by reflecting into it rays of light from a plane or concave 

156 



RETINOSCOPY. 



157 



mirror, and observing the movement which the retinal 
illumination makes by rotating the mirror. 

Suggestion. — Before attempting to practise retinoscopy 
upon the human eye, the beginner is advised to study the 
method upon one of the many schematic eyes to be found 
in the market. 

The principle of retinoscopy is the finding of the point 
of reversal, or myopic far point ; and when an eye is emme- 
tropic or hyperopic, it must be given a myopic far point by 
means of a convex sphere. (Fig. 144.) 

1 METER. 




Fig. 144. 



Advantages of Retinoscopy. — 

1. The character of the refraction is quickly diagnosed. 

2. No expensive apparatus is necessarily required. 

3. The refraction is estimated without the verbal assist- 
ance of the patient. ' 

4. The correction is quickly obtained. 

5. The value of retinoscopy can never be overestimated 
in the young, in the feeble-minded, the illiterate ; in cases 
of nystagmus, amblyopia, and aphakia. 

Axiom. — With an eye otherwise normal except for its 
optic error, and under the influence of a reliable cycloplegic, 
there is no more exact objective method of obtaining its 
refraction than by retinoscopy. 



i 5 8 



REFRACTION AND HOW TO REFRACT. 



The surgeon should wear any necessary correcting 
glasses and have a vision of more than — ; otherwise he 
can never get satisfaction from this method. The surgeon 
should keep his eyes wide open and not hesitate to use his 
accommodation, as it does not have any effect on the result, 
as in estimating the refraction with the ophthalmoscope. 





Fig. 145. 



Fig. 146. 



Author's Mirror with Folding Handle. 

Fig. 145. — Showing central light C, on small mirror B. This is the light the 
patient sees when looking into the mirror, and corresponds in size to the 
one-centimeter opening in screen. D is the folding cap handle to pro- 
tect B when not in use. A is the metal disc. 

Fig. 146. — Shows the light moved to one side as a result of tilting the mirror. 



The patient must have his accommodation under the in- 
fluence of a reliable cycloplegic ; this is imperative. Each 
eye is tested separately, and M the patient has a squint, then 
one eye should be covered while its fellow is being refracted. 
The patient must be comfortably seated and told to look at 



RETINOSCOPY. 



159 



the metal disc of the mirror or the observer's forehead 
above the mirror, and never into the mirror. 

The Retinoscope, or Mirror. — The plane mirror is 2 
cm. in diameter on a round 4 cm. metal disc, with a 2 mm. 
sight-hole at the center, made by removing the silvering, 
and not by cutting a hole through the glass. (See Figs. 
145 and 146.) 

The concave mirror recommended has a 25 cm. focus 
(ten inches) and is 3 y 2 cm. in diameter 
on a metal disc of the same size as the 
plane mirror. The sight-hole is simi- 
lar in size and made in the same way 
as that of the plane mirror. 

The light should be steady, clear, 
and white, and secured to a movable 
bracket. For general use, the Argand 
burner is best. 

The Light-screen, or Cover-chim- 
ney. — For the purpose of intercepting 
the heat this is made of thin asbestos, 
and the iris diaphragm attached to it 
regulates the amount of light desired. 
(See Fig. 147.) 

The room for retinoscopy should be 
darkened and all sources of light except 
the one in use should be excluded. 

Position of Light and Plane Mirror. — These may be 
as close together as 6 inches or as far apart as 6 meters. 
It is a matter of choice with the surgeon himself where he 
prefers to have them. The writer recommends, however, 
having the rays of light come from the 10 mm. opening in 
the light-screen, at about 6 inches to the left and front of 
the surgeon, so that the rays pass in front of the left eye 




Fig. 147. — Author's Iris 
Diaphragm Chimney. 



i6o 



REFRACTION AND HOW TO REFRACT. 



and fall upon the mirror held before the right eye. Some 
surgeons prefer having the light, with the 3 cm. opening in 
the screen, placed over the patient's head or to one side of 
it. (Fig. 148.) The distance between the light and mirror 
will not alter the direction of the rays of light which come 
from the patients eye. 

Position of the Light and the Concave Mirror (Figs. 
148, 149, 150). — As the purpose of the concave mirror in 
retinoscopy is to focus rays of light before they enter the 




METER 



Fig. 148. 



-Light over Patient's Head, and the Observer with Mirror at One 
Meter Distance. 



patient's eye, it is always necessary to have the light and 
mirror widely separated. Usually, the light with the 3 cm. 
opening in the screen is placed to one side or over the pa- 
tient's head, and the surgeon with the mirror is seated about 
one meter from the patient. This will place the focus of the 
25 cm. mirror about 33 cm. in front of the mirror. 

Distance of Surgeon from Patient. — With the plane 
mirror he may approach within a few inches of the patient's 
eye to find the point of reversal, but with the concave mirror 



RETINOSCOPY. 



161 



he must remain at a sufficient distance to have the focus of 
the mirror in front of the patient's eye. 

How to Use the Mirror. — It should be held firmly in 




Fig. 149. — Illustrating High Myopia with a Concave Mirror. 
Rays of light from the lamp (L) are reflected by the mirror (w y ), and form a 
conjugate focus at L/, and the rays from this focal point illuminate the 
retina at L 1 . Corresponding effects result when reflection takes place 
from the mirror at m". The eye (E) behind the mirror recognizes points 
of reversal between the eye and mirror, moving in the same direction to 
that in which the mirror is tilted. 




Fig. 150. — Illustrating Hyperopia with the Concave Mirror. 
The eye (E) recognizes a virtual image behind the eye under examination, so 
that when the mirror (m f ) is focusing the rays from the lamp (L) at I/, 
the upper portion of the retina is illuminated, and vice versa, when the 
mirror [m // ) is focusing the rays at L 2 , the lower portion of the retina is 
illuminated. The retinal illumination moves opposite to that of the mirror. 



the right hand before the right eye, so that the sight-hole 
is opposite to the observer's pupil. The movements im- 



14 



1 62 REFRACTION AND HOW TO REFRACT. 

parted to the mirror must be limited, though they may be 
quick or slow, but never at any time should the mirror be 
tilted more than 2 or 3 mm., otherwise the light will be lost 
from the eye. 

What the Observer Sees, or the general appearance 
of the reflection from the eye. — The reflex from the pupil 
varies in different patients, and is subject to many changes 
as the refraction is altered by correcting glasses, by the 
turning of the patient's eyes, by increasing or diminishing 
the distance between patient and surgeon or the distance 
between the light and mirror, or the strength of the 
light. The amount of pigment in the eye-ground will 
change the general appearance of the reflex, being dim 
in some mulattoes, and very light in the blonde or albino. 
If the refractive error is a high one, the reflex will appear 
dull ; or if a low error, it will appear very bright. If the 
media are not clear, the reflex will be altered accordingly. 
The bright pin-point catoptric images seen on the cornea 
and lens are not parts of the test, and should be avoided or 
ignored. The 1 mm. bright ring of light sometimes seen 
at the edge of the pupil should be avoided by the beginner 
in retinoscopy, as it is an indication of spheric aberration, 
which he will have to consider after mastering other details 
of the method. 

Facial Illumination. — The rays of light reflected from 
the mirror illuminate a portion of the patient's face, and 
always move in the same direction as that in which the 
mirror is tilted, no matter whether the mirror is plane or 
concave. 

Retinal Illumination. — This corresponds to the portion 
of the retina which receives the rays of light reflected from 
the mirror. The retinal illumination is also called " the 
image," "the light area," etc. 



RETINOSCOPY. 1 63 

The Shadow. — This is the non-illuminated portion of 
the retina immediately surrounding the illumination. The 
illumination and shadow are, therefore, in contact ; if the 
illumination changes its place upon the retina by a move- 
ment of the mirror, then the shadow will move also. By 
this change of illumination and shadow we speak of a 
movement of the shadow. 

Where to Look and What to Look for. — Rotating the 
mirror through the various meridians of the eye, the 
observer makes a note of the (i) form, (2) direction, and 
(3) rate of movement of the retinal illumination as he 
watches for them through a four or five millimeter area at 
the apex of the cornea, as this is the portion of the refract- 
ive media in the normal eye that the patient will use 
when the effects of the cycloplegic pass away and the 
pupil regains its normal size. 

Point of Reversal. — To find the point of reversal is the 
underlying principle of retinoscopy. For example, having 
determined with the plane mirror at a distance of one meter 
that the retinal illumination moves with the movement of the 
mirror and a -j-2.50 S. stops all apparent movement (no 
movement of the illumination being seen and the shadow 
having disappeared), the observer knows that his eye is at 
the point of reversal. Or with the concave mirror the 
retinal illumination will move opposite to the movement of 
the mirror and will stop with -j-2.50 S. before the eye. 
The point at which all movement of the retinal illumina- 
tion appears to have ceased is the point of reversal. 

The real movement of the retinal illumination de- 
pends upon the mirror — whether it is concave or plane. 
With the plane mirror the retinal illumination always moves 
with the mirror and the light on the face ; whereas with the 
concave mirror (focusing rays before they enter the eye), 



164 REFRACTION AND HOW TO REFRACT. 

the real movement of the retinal illumination is always 
opposite to that of the mirror. The student should not 
get the real and apparent movements confused, but pay 
close attention to the apparent movement. 

Direction of the Apparent Movement of the Retinal 
Illumination. — With the plane mirror, the apparent move- 
ment of the retinal illumination will be with the mirror and 
with the light on the face as long as the observer is within 
the point of reversal ; but just as soon as the observer is 
beyond the point of reversal, the retinal illumination will 
appear to move opposite to the movement of the mirror 
and opposite to the movement of the facial illumination. 




Fig. 151. 

With the concave mirror the apparent movement of the 
retinal illumination will be with the movement of the mir- 
ror and the light on the face as long as the observer is be- 
yond the point of reversal (Fig. 149) ; but just as soon as 
the observer's eye is within the point of reversal, the retinal 
illumination will appear to move against the movement of 
the mirror and against the light on the face. (See Fig. 150.) 

Rate of Movement of the Retinal Illumination. — This 
is influenced by several factors, but practice will teach the 
observer that when the retinal illumination appears to move 
slowly, the refractive error is a high one, and when it moves 
fast, the refractive error is a low one. 

Figure 151 represents a myopic eye with its far point 



RETINOSCOPY. 



165 



(point of reversal) at R r , and when rotating the mirror, this 
point moves to R" j but if the eye had its far point at F', 
and the mirror was rotated to F /r , then the illumination at 
R', having to move through a smaller arc in the same time, 
appears to move slowly as compared with F', which ap- 
peared to move fast. The same condition is shown in 
figure 152, in which the observer appears to see an erect 
virtual image back of the retina, and R' appears to move 
slowly as compared with F', which appears to move fast. 

Form of Illumination. — A large, round illumination 
may signify emmetropia, hyperopia, or myopia, with or with- 
out astigmatism in combination. Astigmatism is recognized 




Fig. 152. 

by the presence of a band of light, and this band of light 
may be seen before any correcting lens has been placed be- 
fore the eye if the astigmatic error is high ; or it will be 
recognized during the process of neutralization if the error 
is small — i. c, if the astigmatism is of low degree. The pres- 
ence of astigmatism is known, therefore, by the band of light 
or when the illumination appears to move faster in one 
meridian than in the meridian at a right angle. The astig- 
matism is in the meridian of slow movement. 

The apparent difference between the plane and con- 
cave mirror in the direction of movement of the retinal 
illumination. — With the plane mirror the rays of light are 
reflected as if they came from a point just as far back of 



66 



REFRACTION AND HOW TO REFRACT. 



the mirror as the original source of light is in front of it. 
The surgeon's eye behind a plane mirror is, therefore, in 
the path of these rays, and sees that portion of the pupil- 
lary area illuminated to which these rays are directed. 
(See Fig. 153.) 

With the concave mirror the reflected rays come to a 
focus, forming an inverted image of the flame, which be- 
comes the immediate source of light in front of the 
observer's eye. When the concave mirror is tilted, the 
immediate source of light goes in the same direction, but 
with the result that the opposite portion of the pupillary 
area is illuminated. (See Fig. 143.) This shows the 




Fig. 153. 



immediate source of light at \J and mirror tilted downward ; 
the rays proceeding from \J diverge and illuminate the 
upper portion of the pupillary area. Tilting the mirror 
upward, the immediate source of light at \J moves upward 
also (L 2 ), and the lower portion of the pupillary area be- 
comes illuminated. (See also Fig. 149.) 

Rule for Neutralizing Lenses with the Plane Mirror. 
— When the retinal illumination appears to move in the 
same direction as that of the mirror, the observer is within 
the point of reversal and a plus lens must be placed before 
the eye to stop all apparent movement. When the retinal 
illumination appears to move in the opposite direction to 



RETINOSCOPY. 1 67 

that in which the mirror is tilted, the observer is beyond 
the point of reversal, and a minus lens must be placed 
before the eye to stop all apparent movement 

Rule for Neutralizing Lenses with the Concave 
Mirror. — When the retinal illumination appears to move in 
the same direction as that in which the mirror is tilted, the 
observer is beyond the point of reversal, and a minus lens 
must be placed before the eye to stop all apparent move- 
ment. When the retinal illumination appears to move in 
the opposite direction to that in which the mirror is tilted, 
the observer is within the point of reversal, and a plus lens 
must be placed before the eye to stop all apparent move- 
ment. 

Rule for neutralizing lenses, no matter whether the 
mirror is plane or concave. — When within the point of 
reversal, use a plus lens, and when beyond the point of 
reversal, use a minus lens. 

Application of Retinoscopy in Emmetropia (Fig. 
153). — Rays of light proceed parallel from an emmetropic 
eye under the influence of a cycloplegic, and if a -f I S. 
is placed in front of such an eye, the rays will converge 
and form a point of reversal at I meter distance, and the 
observer at this point will not be able to see any move- 
ment of the retinal illumination. The same result would 
have been obtained at % of a meter if a +3 S. had been 
used, or at 4 meters if a -(-0.25 S., or at yi of a meter 
if a -j-2 S. had been used, etc. 

In taking the patient from the dark-room to test his 
vision at 6 meters, an allowance must always be made for 
the distance from the patient's eye at which the point of 
reversal was found. If at % of a meter, 3 S. must be de- 
ducted from the lens used ; if at y£ of a meter, 4 S.; if at 
6 meters, nothing, or 0.12. 



1 68 REFRACTION AND HOW TO REFRACT. 

Application of Retinoscopy in Hyperopia. — The 

same conditions hold good in hyperopia as in emmetropia. 
If a +4 S. gives ^ a point of reversal at one meter, then 
I S. must be taken from the 4 S. to give the eye parallel 
rays of light, or infinity vision. If a +4 S. gave a point 
of reversal at 2 meters, then 0.50 S. would have to be 
deducted from the 4 S. for the infinity correction, which 
would be +3- 50 S. 

Application of Retinoscopy in Myopia. — Rays of light 
from a myopic eye come to a focus at some point inside of 
infinity, and if the surgeon so desires, he may approach such 
an eye from a distance of six meters, until he finds a point 
where the retinal illumination ceases to move (where it does 
not appear to move) ; and then, measuring this distance from 
the eye under examination, he can quickly calculate the 
amount of the myopia. This can not be done with the 
concave mirror if the myopia is more than 2 S. If the 
reversal point is at 4 meters, 3 meters, 2 meters, 1 meter,. 
y 2 of a meter, ^ of a meter, or % of a meter, then the 
myopia would be 0.25 S., 0.33 S., 0.50 S., 1 S., 2 S., 3 S., 
4 S., respectively. 

If the surgeon will always refract the patient's eyes so 
that he gets the point of reversal at 1 meter distance, he 
will have the following rule to guide him — i. e. : 

To add a — 1 sphere to the dark-room correction, no 
matter what that may be. For example : 

Dark-room, 0.00 40.25 S. -fo.5oS. +0.75S. +1. 00S. +1.25S. 
Add, . . . — 1.00S. — 1.00S. — 1.00S. — 1.00S. — 1.00S. — 1.00S. 



Infinity, . . — 1.00D. — 0.75D. — 0.50 D. —0.25 D. 0.00 -fo.25D. 

Application of Retinoscopy in Astigmatism. — If the 

surgeon has mastered retinoscopy in hyperopia and myopia, 
he should not have any difficulty in pursuing exactly the 




RETINOSCOPY. 1 69 

same course irr cases of astigmatism. As already stated, 

the presence of astigmatism is diagnosed by the presence 

of a band or ribbon-like streak of bright illumination which 

extends across the pupillary area. 

(See Fig. 154.) This band of light 

may be seen before any neutralizing 

lens is placed in front of the eye, 

if the astigmatism is in excess of 

the spheric correction, as in the 

following formula : 

+0.75 sph. C+4.50 cyl. axis 105 degrees. FlG - I 54-— Band of Light. 
, X ■ ■ 1 • * j Astigmatism Axis 90 de- 

— 1. 00 sph. 3 — 5-°° cyl- axis ID 5 degrees. ^ 

grees. 

Or the presence of astigmatism 
may not be recognized until after a sphere has been placed 
in front of the eye, as in one of the following formulas : 

-(-4.50 sph. 3 +°-75 c yl- ax i s 75 degrees. 
— 5.00 sph. 3 — 1 -°° c yl- ax i s l &° degrees. 

In refracting cases of astigmatism with the retinoscope, 
all the surgeon has to do is to refract the meridian of least 
ametropia first, and then the meridian of greatest ametropia. 
Taking the following formula : 

+2.50 sph. 3 \ 1. 00 cyl. axis 90 degrees ; 

in the dark-room a +3.50 S. would make all movement 
cease in the vertical meridian, at one meter distant ; but 
when the mirror is tilted in the horizontal meridian, there 
would be seen a band of light extending across the pupil 
on axis 90 degrees. Then, substituting +4.50 S. for the 
3.50 S., all movement will cease in the horizontal meridian, 
a +4.50 S. neutralizing the horizontal meridian. The 
difference between these two spheres is 1 D., which is the 
amount of the astigmatism. In neutralizing astigmatism 
the writer advises using spheres, and after each meridian 
'5 



I70 REFRACTION AND HOW TO REFRACT. 

has been refracted, to make the cylindric correction, and 
prove it, if so desired. 

Axonometer. — To find the exact axis subtended by the 
band of light while studying the retinal illumination, when 
the meridian of least ametropia has been corrected, the 
writer has suggested a small instrument, which, for want of 
a better name, he has called an axonometer. This is a 
black metal disc, with a milled edge, 1 y 2 mm. in thickness, 
of the diameter of the ordinary trial-lens, and mounted in 
a cell of the trial -set. It has a central round opening, 12 




Fig. 155. 

mm. in diameter — the diameter of the average cornea at its 
base. Two heavy white lines, one on each side, pass from 
the circumference across to the central opening, bisecting 
the disc. To use the axonometer, place it in the front 
opening of the trial -frame, and with the patient seated erect 
and frame accurately adjusted, so that the cornea of the eye 
to be refracted occupies the central opening. As soon as 
that lens is found which corrects the meridian of least 
ametropia, and the band of light appears distinct, turn the 



RETIXOSCOPY. 171 

axonometer slowly until the two heavy white lines accu- 
rately coincide, or appear to make one continuous line with 
the band of light. (See Figs. 155, 219.) 

The degree mark on the trial-frame to which the arrow- 
head at the end of the white line then points is the exact 
axis for the cylinder. 

Application of Retinoscopy in Mixed Astigmatism. — 
Here the dark-room correction (after making deductions for 
the distance of the point of reversal) will show one meridian 
myopic and the other, at right angles to it, as hyperopia If 
the astigmatism is more than one diopter in each meridian, 
the surgeon will diagnose in the dark-room the condition 
of mixed astigmatism by opposite movements in the me- 
ridians of minimum and maximum ametropia. 

Application in Irregular Astigmatism. — This condi- 
tion is either in the lens or cornea, usually in the latter. 
The reflex is more or less obscured by areas of darkness, 
which make it extremely difficult to study the refraction, 
and the observer will have to change his distance repeat- 
edly to find clear spaces as close to the center of the pupil 
as possible, as it is this portion of the pupillary area that 
the patient will see through when the mydriatic effect passes 
away. The kaleidoscopic picture obtained by moving the 
mirror so as to describe a circle at the periphery of the 
pupillary space is quite diagnostic of the corneal condition. 
Whatever correction is obtained should be kept for reference 
in a postcycloplegic manifest refraction, as it will not always 
do to order the glasses while the eye has its pupil dilated. 
The patient may choose a slightly different correction in 
such cases, after the pupil regains its accustomed size. 

Irregular Lenticular Astigmatism. — This is often more 
uniform than the corneal variety, and is characterized by 
faint striae in the lens, pointing in toward the center. If 



172 



REFRACTION AND HOW TO REFRACT. 




the striae are not very faint, they may be recognized with the 
ophthalmoscope, even before any cycloplegic has been used. 
Scissor Movement (see Fig. 156). — This is a condi- 
tion in which two bands of light are present, usually in 
the horizontal meridian or inclined a few degrees therefrom. 
Tilting the mirror in the vertical meridian, a band of light 
is seen to come from above and to meet another band, which 
comes from below ; while these two bands are approaching, 
the dark space between them gradually disappears, until the 

two bands unite and form one band 
across the pupil in or approximat- 
ing the horizontal meridian. This 
movement of the bands is likened 
to the action of the blades of a pair 
of scissors, and hence the name. 
To refract a case of this character, 
the observer must proceed slowly 
and endeavor to neutralize the 
horizontal meridian first, and then 
add minus cylinders with the axis corresponding to the 
axis of the two bands. The resulting prescription should 
also be a plus sphere with a minus cylinder, the cylinder 
of less strength than the sphere, as the condition is not one 
of mixed astigmatism, the patient preferring this combina- 
tion, as a rule. 

Conic Cornea. — In this condition the observer is im- 
pressed at once with the bright central illumination, which 
usually moves opposite to the movement of the peripheral 
illumination. The best way to neutralize a case of this 
character is to proceed as in a case of irregular astigmatism. 
The observer should also be on the lookout for a band of 
light in this central illumination, as most of these cases are 
astigmatic. 



Fig. 156. — Scissor Move 
merit. 



RETINOSCOPY. 



173 



Spheric Aberration. — This is of two kinds — -positive 
and negative. (See Figs. 157 and 158.) In the positive 
form the peripheral refraction (A, A, that at the edge of the 
pupil) is stronger than the central (B, B) ; the reverse of 
this condition, negative aberration, is seen in conic cornea. 
These two varieties of refraction should not worry the 
observer, as most of the peripheral aberration is covered 
up by the iris when mydriasis passes away, and, therefore, 
is not of any great moment, except in conic cornea. 




Fig. 157. — Positive Aberration. 




Fig. 158. — Negative Aberration. 



The Reisner Retinoscope (Figs. 159 and 160). — This 
instrument combines the mirror with an axis finder. The 
metal disc, on which the mirror is secured by means of a 
coiled spring at one point only, has a milled edge like that 
of an ophthalmoscope. (See Fig. 159.) At the junction 
of the metal disc with the handle is a small push-button 
which has a short metal pointer which extends upward and 
beneath the mirror. Figure 160 is a back view of the in- 
strument, and shows the degree-marks of half a circle and 
an index or pointer. When beginning the examination, 



174 REFRACTION AND HOW TO REFRACT. 

this instrument may be used just the same as any other 
retinoscope, but as soon as the surgeon sees a band of light 
then he presses the push-button to see if the mirror rotates 
with its axis corresponding to the long measurement of the 
band of light. If this is not so, then the mirror must be 
turned by means of the milled edge, using the index-finger 
as in turning the disc of an ophthalmoscope. When the 
axis of rotation of the mirror corresponds with the long 




Fig. 159. — (One-half size.) Fig. 160.- (One-half size.) 



axis of the band of light, then the pointer on the back of 
the instrument indicates the axis of the astigmatism, and 
now all the surgeon has to do as he makes the changes in 
the lenses is to press the button only. The handle of the 
retinoscope is now held perfectly still and the upper portion 
of the disc rests firmly against the observer's brow. The 
merits of this ingenious instrument are as follows : The 



RETINOSCOPY. 



175 



handle is held perfectly still, the mirror alone does the 
moving; the sight-hole is always in front of the pupil; the 
axis of the astigmatism is carefully recorded. 




^ 



Fig. 161. — (Two-thirds size.) 



The Luminous Retinoscope (Figs. 161 and 162). — 
DeZe?ig Patent. — The latest improvement in retinoscopes is 



I76 REFRACTION AND HOW TO REFRACT. 

the luminous instrument here described. This instrument 
is the author's plane mirror with the electric light attach- 
ment. (Fig. 161.) The filament is contained in a tube 
placed at an angle of 45 degrees with the handle and the 
mirror is correspondingly tilted at an angle of 22 degrees. 
The light from the filament passes divergently to a strong 
convex lens which renders the rays less divergent as they 
fall upon the mirror, and from the mirror the rays pass 
divergently to the patient's eye. (See Fig. 162.) This in- 
strument has innumerable points of merit: It does away 
with any use of gas or lamp or cover chimney; the ob- 
server is not annoyed with the heat from the gas or lamp ; 
the observer does not have to move the light or bracket 
when changing from one distance to another as when work- 
ing with the gaslight close to the mirror; the electric wires 
(cords) carrying the current to the filament are of sufficient 
length to give the observer two meters of space in which 
to practise the method; the brilliancy of the illumination 
can be made most intense or diminished very materially 
with a convenient rheostat ; the size of the divergent pencil 
may also be controlled by adjusting the condensing lens at 
the end of the tube. (See Appendix.) 



CHAPTER VII. 
MUSCLES. 

Examination of the External Eye Muscles. 

General Considerations. — When the retinal image of an 
object is situated exactly on the fovea, the eye is said to 
" fix " the object. 

Normally, when both eyes " fix " the object, each eye 
has an image of the object on its fovea, and these foveal 
images or impressions are transmitted to the brain and fused 
as one image in the visual centers. This condition is spoken 
of as equipoise, or orthophoria, and the eyes are said to be 
in equilibrium, or to balance. Whenever one eye alone fixes 
an object, and the fellow-eye receives the image of the same 
object on a part of its retina distant from the fovea, then 
the brain takes note of two separate impressions, and this 
condition is spoken of as double vision (diplopia). 

(a) The image of an object formed upon the retina above 
the fovea is projected downward — i. e. y objects situated 
below the horizontal line of vision are recognized by that 
portion of the retina above the fovea. 

iti) The image of an object formed upon the retina below 
the fovea is projected upward — i. e. y objects situated above 
the horizontal line of vision are recognized by that portion 
of the retina below the fovea. 

(c) The image of an object formed on the retina to the 
nasal side of the fovea is projected toward the temporal 
side — i. e. y objects to the temporal side have their images 
formed upon the nasal portion of the retina. 

177 



l 7 8 



REFRACTION AND HOW TO REFRACT. 



(d) The image of an object formed on the retina to the 
temporal side of the fovea is projected toward the nasal side 
—i. e., objects to the nasal side have their images formed 
upon the temporal portion of the retina. 

Homonymous Diplopia (Greek, 6p.'6vu/io^ ; from 6;±6q, 
same, and ovu/ia, name). — Figure 163 shows the right 




Fig. 163. 



eye (R) fixing upon the object (O), but the left eye is 
turned inward, so that rays from O fall upon its retina to the 
nasal side of the fovea (M), and are projected outward to 
the temporal side ; the result is that the left eye sees a false 
object to the left of the real object. This condition of the 
objects is spoken of as homonymous diplopia. 



MUSCLES. 



179 



Heteronymous Diplopia (Greek, irepos, other ; and ovupa, 
name). — Figure 164 shows the right eye fixing the object 
(O), but the left eye is turned outward, so that rays from O 
fall upon the retina to the temporal side of the fovea and 
are projected to the nasal side, with the result that the left 
eye sees a false object 
to the right of the real 
object. This condi- 
tion of the objects is 
spoken of as heter- 
onymous or crossed 
diplopia. 

Hyperphoria 
(Greek, u-sp f over, 
above ; yopelv, to 
tend). — In the con- 
sideration of vertical 
diplopia, — which is 
always a condition 
of crossed diplopia, 
never homonymous 
diplopia, — the eye 
which is deviated up- 
ward is spoken of as 
the hyperphoric eye, 
and necessarily its 
image must be lower 

than its fellow. For instance, if the left eye fixes an object 
and the right eye is turned upward, the rays of light from 
the object would fall upon the upper part of the retina of the 
right eye, and would be projected downward below the true 
object ; and this position of the right eye is spoken of as 
right hyperphoria. Or if the right eye fixes an object and 




O'M 



Fig. 164. 



ISO REFRACTION AND HOW TO REFRACT. 

the left eye sees a false object below, then the position of 
the left eye is spoken of as left hyperphoria. Unfortunately, 
in hyperphoria (unless from paralysis) the position of the 
eyes does not tell whether the right superior rectus is too 
strong and the left inferior rectus too weak, or the left supe- 
rior rectus too weak and the right inferior rectus too strong. 

Muscle Phorometry. — Testing the power of the ocular 
muscles. 

Abduction. — The power of the external recti muscles to 
turn the eyes outward. The patient is comfortably seated 
and told to look at a point of steady light at a distance of 
about 6 meters, slightly below the level of his eyes, never 
above the level. In this position prisms with their bases in- 
ward are placed in front of one or both eyes until the 
patient says he sees two lights very close together. The 
strength of the prism or prisms thus placed before the 
eyes which will just permit the eyes to see one object 
and if increased would produce diplopia, represents the 
power of the external recti muscles. This is spoken of as 
the power of abduction, and is abbreviated Abd. For 
example, if with 7 centrads, base in, before the eyes there 
are two lights, and with 6 centrads there is only one light, 
then 6 centrads would represent the amount of the abduc- 
tion. In other words, in the case supposed, as long as 
there is less than 7 centrads before the eyes, base inward, 
the external recti muscles can overcome their effect, but as 
soon as a prism stronger than 6 centrads is used, then the 
external recti muscles can not counteract the effect, and 
diplopia is the result. 

Adduction. — The power of the internal recti muscles to 
turn the eyes inward. The power of the internal recti is 
tested in the same way as the external, except that the 
prism is placed base outward. This is spoken of as adduc- 



MUSCLES. 1 8 J 

tion, and is abbreviated Add. For example, if with 19 
centrads, base out, before the eyes two lights are seen, and 
with 1 8 centrads only one light, then 1 8 centrads represent 
the power of adduction. In other words, as long as there is 
a prism of 18 or less than 18 centrads before the eyes, base 
outward in this case, the internal recti muscles can over- 
come the effect ; but as soon as a prism stronger than 1 8 
centrads is used, then the internal recti muscles can not 
counteract the effect, and diplopia is the result. It must 
be remembered that the internal and external recti are 
antagonistic, and that the muscles of the two eyes are 
tested together. The relative power of adduction to abduc- 
tion has been variously estimated, but most authorities are 
agreed that adduction is about three times that of abduc- 
tion, or about 3 to 1 — that is to say, in eyes with normal 
muscle balance, if adduction is represented by 18 centrads, 
then abduction should be 6 ; or if adduction is represented 
by 24 centrads, then abduction should be 8 centrads ; or if 
adduction is 12 centrads, then abduction should be 4 cen- 
trads, etc. 

Sursumduction. — This is the power of the eyes to fuse 
two images when one eye has a prism placed base up or 
down before it. For example, if a 3 y 2 centrad prism is 
placed base up or down before either eye and diplopia 
results and persists, and then a 3 centrad is substituted and 
there is no diplopia, then the eyes have overcome the effect 
of the prism and the amount of the sursumduction is said 
to be 3 centrads. This test for sursumduction is made at 
the same distance as in testing the lateral muscles. In 
health the power of the superior and inferior recti muscles 
is, as a rule, the same — that is to say, they antagonize each 
other equally. The power of the superior recti is spoken of 
as supraduction, sursumvergence ; and that of the inferior 
recti, as infraduction, deorsumvergence. 



1 82 REFRACTION AND HOW TO REFRACT. 

Muscular Imbalance. — Whenever there is any disturb- 
ance in the power, strength, or force of the ocular muscles, 
the condition is no longer one of equipoise, or equilibrium, 
or muscle balance, but is spoken of as muscular imbalance 
(heterophoria). From this statement it must not be sup- 
posed that the two eyes can not simultaneously "fix" an 
object, any more than it must be supposed that a hyperopic 
eye can not see or have — vision without correcting 
glasses. 

Just as in hyperopia distant vision may be made clear 
by the effort of accommodation, so in muscular imbalance 
the visual axes can be directed to one point of fixation by 
increased innervation. Muscular imbalance is subdivided 
into two classes — insufficiency and strabismus. 

The following nomenclature of muscular anomalies, sug- 
gested by Stevens, of New York, is in common use : 
Orthophoria, perfect muscle balance, equipoise, or binocular 

equilibrium. 
Orthotropia, perfect binocular fixation. 

Heterophoria, imperfect binocular balance, or imperfect bin- 
ocular equilibrium. 
Heterotopia, a squint or decided deviation or turning from 

parallelism. 
Hyperphoria, a tendency of one eye to deviate upward. 
Hypertropia, a deviation of one eye upward. 
Esophoria, a tendency of the visual axes to deviate inward. 
Esotropia, a deviation of the visual axes inward. 
Exophoria, a tendency of the visual axes to deviate outward, 
Exotropia, a deviation of the visual axes outward. 
Hyperesophoria, a tendency of the visual axis of one eye 

to deviate upward and inward. 
Hyper esotropia, a deviation of the visual axis of one eye 

upward and inward. 



MUSCLES. 183 

Hypcrexophoria, a tendency of the visual axis of one eye 

to deviate upward and outward. 
Hyper exotropia, a deviation of the visual axis of one eye 

upward and outward. 

Insufficiency. — Also called latent deviation, hetero- 
phoria, or latent squint. This may be defined as the con- 
dition in which there is a tending or tendency of the visual 
axes to deviate from the point of fixation ; this may be slight 
or transitory. 

Causes of Insufficiency. — The chief cause of insuffi- 
ciency is some form of ametropia. Another cause may be 
an anatomic defect of one or more of the ocular muscles 
themselves, or a weakness of the muscle or muscles indi- 
vidually, or as a result of some systemic weakness. The 
ocular muscles often sympathize with the economy. 

Symptoms of Insufficiency, or Muscular Asthenopia. 
— Accommodative and muscular asthenopia are intimately 
associated, and the latter is so often the companion of the 
former that they produce symptoms which are identical in 
both and make it difficult to draw any sharp line of demar- 
cation between the two. In muscular asthenopia, how- 
ever, the patient complains that the eyes "become weak" 
or "tired" after any prolonged use, and that this is espe- 
cially apt to occur by artificial light ; that nearby 
objects (reading, writing, or sewing) grow dim ; that the 
words "seem to jump," or the "letters run together," and 
in some cases occasionally, and in others more frequently, 
objects appear double for a moment. Sometimes one of 
the eyes feels as if it was turning outward or inward. 
There are innumerable reflex symptoms, dizziness, nausea, 
vomiting, fainting, and, in some instances, "all becomes 
dark for a minute." Such patients often become very 
anxious, fearing sudden blindness, etc. 



184 REFRACTION AND HOW TO REFRACT. 

Diagnosis or Tests for Insufficiency (Heterophoria). — 

Before taking up the individual tests for insufficiencies, it is 
well for the observer to study the movements or excursions 
of the eyes ; and to do this the patient, with his head erect 
and steady in one position, fixes with his eyes the point of 
a pencil held in the hand of the observer at about thirteen 
inches distant. The pencil is moved from left to right and 
from right to left, and upward and downward ; as this is 
done, the surgeon should watch closely to see that each 
eye has a normal mobility and the two eyes move together. 
From a central point of fixation the eyes should move 
inward about 45 degrees, outward 45 or 50 degrees, 
upward about 40 degrees, and downward about 60 degrees. 
The tropometer of Stevens will estimate the limit of motion 
of each eye separately, but if there is a defect in mobility, 
the surgeon may recognize it by comparing the distance of 
the corneal edge in each eye from a certain definite fixed 
point ; for instance, whether the lid margins encroach 
equally upon the cornea or have equal intervals between 
cornea and lid edges. 

The Cover Test. — The patient is told to look at the 
point of a pencil held in the hand of the surgeon on a level 
with the patient's eyes in the median line, and distant about 
eighteen inches, or at an object six meters distant. While 
the eyes fix the point of the pencil or distant object, the 
surgeon covers one eye with a small card, and a moment 
later quickly withdraws it and observes the position and 
movement of the eye which he has just uncovered ; if it 
moved inward toward the nose to fix the point of the pen- 
cil, then there must have been an outward tendency of that 
eye when under cover ; in other words, the external muscles 
must have been strong or the internal weak. If the eye 
thus released from the cover had moved outward toward 



MUSCLES. I85 

the temple to fix the point of the pencil, then the external 
recti must have been weak or the internal strong. 4 If the 
eye released from cover goes up to fix. then the fellow-eye 
deviates upward, and vice versa. This test is not always 
reliable, and yet it may be a guide to further study. 

The Fixation Test. — Instead of covering one eye, as in 
the previous test, the patient " fixes " the point of the pencil 
as it is slowly advanced in the median line toward the nose, 
up to within four inches, if necessary. During this advance 
of the pencil, if there is a weakness of the interni, the eye 
with the weaker internus is the one which will usually 
deviate outward. 

To Determine Lateral Insufficiency. — The condition 
in which there is either a tendency for the visual axes to 
deviate outward (exophoria), or a tendency for the visual 
axes to deviate inward (esophoria). Proceed by producing 
vertical diplopia. Place a ten centrad prism base down 
before one eye, — for instance, the right eye, — and have the 
patient look at a point of light on a level with his eyes at a 
distance of six meters. He will see two lights, one above 
the other ; the upper light must belong to the right eye, 
because the prism before the right eye bent the rays down- 
ward. If one light is directly above the other, then the 
condition is presumably one of equilibrium or equipoise. 

If the upper light, however, is to the right, then the 
visual axes deviate inward (esophoria). The amount of 
the esophoria (insufficiency of the external recti) is repre- 
sented by that prism placed base outward before the left 
eye which will bring one light directly above the other. If 
the upper light had been to the left, then there would have 
been a tendency of the visual axes outward (exophoria, 
insufficiency of the internal recti), and the amount of the 
exophoria is represented by the strength of prism placed 



1 86 REFRACTION AND HOW TO REFRACT. 

base inward which will bring one light directly above the 
other. 

To Determine Vertical Insufficiency (Hyperphoria). — 

Proceed by producing lateral diplopia. Place a ten centrad 
prism base inward before the right eye, and have the patient 
look at a point of light, as in testing for lateral insufficiency. 
(It is always well to have the point of light just in front of 
a large piece of black felt cloth tacked upon the wall.) 
If the two lights which the patient sees are on a horizontal 
line, then the condition is presumably one of equipoise. But 
if the right light is lower than the left, there is a tendency 
of the visual axis of the right eye to be higher than its 
fellow. As to which muscle is at fault, this test will not 
tell, and Stevens' tropometer will have to be used. The 
amount of the deviation is represented by the strength of 
prism placed base down before the right, or upward before 
the left eye which will bring the two lights into a horizontal 
line — i. e. f on a level. 

To Determine Lateral Insufficiency at the Reading 
Distance. — Have the patient look at a black dot, with a 
black line two or three inches long running perpendicularly 
through it, at a distance of about thirteen inches. This is 
known as the line-and-dot test of von Graefe, and on a 
larger scale may also be used in the previous tests. A prism 
of seven or eight centrads is placed, with its base down, in 
front of the right eye. If the patient sees two dots exactly 
one above the other on one line, there is not supposed to be 
any insufficiency. If, however, there are two lines and 
two dots, and the upper dot is on the right, there is in- 
sufficiency of the externi (esophoria) for near. The amount 
of the insufficiency is represented by the strength of prism, 
placed base outward, before the left eye which will bring 
the two dots exactly on one line. If the upper dot is to 



MUSCLES. 187 

the left, then there is insufficiency of the interni (exophoria) 
for near, and the amount of the insufficiency is represented 
by the strength of prism placed base inward over the left 
eye which will bring the two dots, one above the other, on 
one line. 

Another method for testing lateral insufficiency at the 
reading distance of 13 inches is to have a card about 6 
inches square, and on this card to draw a heavy black line 
about 3 inches long; this line is to be horizontal. At 
the middle of the horizontal line draw a heavy black line, 
one-half an inch long, extending vertically from the hori- 
zontal line ; this short vertical line to be capped with an 
arrow-point. The horizontal line is divided off into equal 



Esophoria 



IS I* 13 12 II 10 9 8 7 6 5 4 3 2 1 

I I I I I I I ' l 1 I I 1 1 



Exophoria 



2 3 4 5 6 7 8 9 10 II 12 13 14 15 

J L_J I I I I I I I L_J I I 



- Fig. 165. — Scale for Testing Lateral Insufficiency at 13 inches. 

spaces, each 3^ millimeters apart and numbered from I 
to I 5 each side of the arrow ; those to the left of the arrow 
are marked "esophoria," and those to the right of the arrow 
are marked "exophoria." (See Fig. 165.) 

To use this method, a prism of 8 centrads is placed base 
down before the right eye ; this doubles the scale vertically; 
the upper scale belongs to the right eye. The number 
and the word in the upper scale to which the arrow in the 
lower scale points, is the approximation in centrads of the 
amount of the esophoria or exophoria. For instance, if 
the lower arrow points to figure 9 in the upper scale to the 
right of the upper arrow, that is, the word "exophoria," 



188 



REFRACTION AND HOW TO REFRACT. 



then there is approximately 9 degrees of "exophoria" at 
this distance of 1 3 inches, the distance at which this scale 
is intended to be used. 

These tests for insufficiencies should always be made be- 
fore estimating the refraction, and also after the correcting 
lenses are carefully placed, with their optic centers, before 
the eyes. 

To avoid confusion in making these tests, when a point 
of light is used as the fixing object, it is customary to place 
a piece of plane dark red glass before one eye, so that the 





Fig. 166.— Maddox Rod. 



Fig. 167. 



red light always corresponds to the eye with the red glass. 
Or a Maddox rod (Fig. 166), white or red, may be used 
for the same purpose. This may be a series of rods (see 
Fig. 167) placed in a metal cell of the trial-case, and the 
eye, looking through it at the light, will see the image of 
the flame distorted into a streak of broken light. A strong 
-f cylinder from the trial-case will answer the same pur- 
pose. As the rod refracts rays of light opposite to its axis, 
the eye will see a streak of light in the reverse meridian to 




MUSCLES. 159 

that in which its axis is placed. To expedite the deter- 
minations, the rotary prism of Cretes or the revolving 
prisms of Risley may be employed. This latter apparatus 
(see Fig. 168) is composed of 
two superimposed prisms of 1 5 
centrads each, and mounted in a 
milled-edged cell of the size of 
the trial-lens. By means of a 
milled-edged screw these prisms 
are made to revolve so that in 
the position of zero they neutral- 
ize each other, and when rotated 
over each other the prism FlG l68 

strength gradually increases un- 
til the bases of the prisms come together and equal 30 
centrads. The strength of the prism employed is indicated 
by an index on the periphery of the cell. 

Stevens ' Phorometer. — This is a very convenient appa- 
ratus, composed of two 4-degree prisms placed in a frame 
3 y 2 inches from the eyes, which with an attached lever can 
be rotated so as to test the strength of the vertical and 
lateral muscles. Indexes and letters at the periphery of 
the frame record the character and degree of the insuffi- 
ciency. (See Fig. 169.) 

Treatment of Insufficiencies. — As ametropia is the 
most common cause of insufficiency, the first consideration 
must be to select the proper correcting glasses. After this 
has been accomplished, if the insufficiency still persists and 
the patient is not comfortable, then the muscles should re- 
ceive careful attention, and their condition be studied from 
every point of view. The patient's general health should 
be looked after, and if at all defective, must have remedies 
prescribed for its improvement. In some instances the 



190 



REFRACTION AND HOW TO REFRACT. 



patient may have to give up any close application of the 
eyes for a time and pursue an out-door life. Operative 
interference (tenotomy) must not be entertained until all 
known means for the relief of the muscular asthenopia 
have been exhausted. 

The prescribing of prisms, as a fixed rule, for permanent 
use, which correct insufficiency, except in vertical errors, is 
often a serious mistake on the part of the surgeon, as in 
most instances they often do more harm than good by in- 
creasing the difficulty. Internally, sedatives will frequently 



Phorometer 

— - Revolving 
Trial Frame 

f ev0lvin £ tfFSiP^Taii^ Revolving 

Bb^M^&mWfo Rotary Prism 




Fig. 169. 



give great satisfaction and permanent relief. The writer is 
partial to the use of bromids with small doses of the iodid 
of potash three or four times a day. The modus operandi 
is not clear. The only guide that can be suggested is to 
use sedative treatment and rest of the eyes whenever there 
is a congestion of the choroid and retina and when the 
ophthalmoscope shows the nerve edges hazy, the retina 
woolly, etc. In another class of patients the internal use 
of nux vomica is the treatment par excellence, and it acts 
best in those cases where the nerve edges and the eye- 
ground in general appear clear and free from irritation. 



MUSCLES. I9I 

To use nux vomica it must be given in the form of the 
tincture and increased, one drop at each dose, until the 
patient becomes quite tolerant of if, taking as high as thirty, 
fort}*, or even fifty drops three times a day, and then the 
dose is gradually diminished. Nux vomica does not seem 
to do well in cases in which the bromids are indicated as 
above, and vice versa. (De Schweinitz.) 

Treatment of Insufficiency of the Internal Recti. 
— Because the tests for heterophoria at 6 meters show an 
ability on the part of the patient to maintain equilibrium, it 
must not be supposed that there may not be an insufficiency. 
The normal ratio of adduction to abduction should be taken 
into consideration in every instance before coming to any 
such conclusion. 

After the proper correcting glasses have been prescribed 
and the patient's general health looked after, attention, if 
necessary, should be directed to strengthening the weak 
muscles ; and to do this they must be given a certain amount 
of systematic exercise, known as ocular gymnastics. That 
success shall result from ocular gymnastics means perse- 
verance on the part of the patient and the exercises system- 
atically executed. There are two methods of procedure : in 
cases ofexophoria — Dr. George M. Gould, "Med. News," 
Nov. 18, 1893 — 

1. Have the patient "fix" the point of a pencil, or the 
end of his finger held at arm's length, and slowly draw 
it toward the bridge of the nose. If diplopia results while 
doing this, the exercise should cease, and be repeated from 
the original distance. This is a very convenient exercise 
and should be practised several times a day and a number 
of times at each sitting. 

2. Prism Exercises. — The patient is placed, standing, 
about a foot or two from a point of steady light, on a level 
or slightly below the level of the eyes, and told to look at 



192 REFRACTION AND HOW TO REFRACT. 

it, and at nothing else. In this position a pair of weak 
prisms, bases out, in a trial-frame are placed in front of his 
eyes. 

Then he is told to walk slowly backward as he keeps 
his eyes fixed on the point of light. Should diplopia de- 
velop at any distance short of 20 feet, then he is to raise 
the prisms, go back to his original position, and start over 
again. Repeating this a number of times in the surgeon's 
office, it will be found, in most instances, that at the first 
practice a pair of 5 A or 10 A can be overcome at a distance 
of 20 feet. When the distance of 20 feet from the light is 
reached without developing diplopia, the patient is instructed 
to slowly count 20 or 30 (keeping the light single during 
this time), then raise the prisms (gazing at the light), and to 
slowly count 20 or 30 again. This exercise is repeated 
three or four times a day and a number of times at each 
practice. A prescription is given for such a pair of square 
prisms with a convenient frame to wear over the patient's 
glasses. These exercises should, as a rule, be conducted 
with the patient wearing his correction. Instead of the 
prism-frame, the patie'nt may hold the square prisms with his 
hands ; but these are tiresome to hold, and for general use 
the prism-frame, if not too heavy, is preferable. After a 
few days' practice at home, the patient returns, and stronger 
prisms which will permit the patient to maintain single 
vision are ordered. This practice with stronger and stronger 
prisms is repeated until the patient is able to overcome 
prisms greatly in excess of the normal ratio of adduction 
to abduction. It is often well to develop the power of the 
internal recti to three or four times the strength of the ex- 
ternal recti ; for when the exercises are stopped, some of the 
strength of adduction will rapidly disappear. 

It has been incidentally mentioned that prisms should 



MUSCLES. I93 

not be prescribed in combination with the ametropic cor- 
rection for the treatment of insufficiency, and yet there is 
an occasional exception to this statement in cases which 
must have prompt, though temporary, relief Occasion- 
ally, the relief may be permanent ; but this will not hap- 
pen very often. When ordering prisms for such a case, it 
is best to prescribe them in the form of hook fronts, so that 
they may be thrown aside at any time. In hyperphoria the 
full prismatic correction (except in cases of presbyopia) is 
seldom ordered, — only about two-thirds of it, and this is 
divided between the two eyes — base down before one, and 
base up before the other. 

Treatment of Insufficiency of the Extern! (Esophoria). 
— As esophoria is a tendency of the visual axes to deviate in- 
ward, it will be found that patients with this form of insuffi- 
ciency suffer very little, if at all, when using the eyes at near 
work ; their chief discomfort arises from using the eyes for 
distant vision. The " shopping headache," the " opera head- 
ache," the " train headache," may be due to this form of in- 
sufficiency, but it is not so apt to cause discomfort if the 
ametropic correction is worn constantly. In other words, if a 
hyperope does not wear his distance correction and accom- 
modates at the same time that he endeavors to maintain equi- 
poise (relative hyperopia), he may at times suffer severely. 
If the symptoms of muscular asthenopia persist after pre- 
scribing the ametropic correction, then prisms, bases out, 
may be prescribed as hook fronts to be worn over the con- 
stant correction when using the eyes for distance. It has 
been the writer's experience that esophoria of two, three, 
or four degrees seldom gives the possessor any discomfort 
whatever. Prism exercises for esophoria give very little 
benefit, and are often a waste of time ; yet they should be 
tried thoroughly if the case appears to demand it, 
17 



194 REFRACTION AND HOW TO REFRACT. 

Treatment of the Insufficiency of the Superior and 
Inferior Recti. — Having prescribed the ametropic correc- 
tion, an attempt should be made to strengthen the weak 
muscles by prism exercises — prism base down before one 
eye, and base up before the other eye. While this does not 
often give satisfactory results, yet it should be tried in each 
instance. If prism exercises do not correct the difficulty, 
then prisms which overcome most of the insufficiency should 
be prescribed for constant use. Failing in this second 
attempt with prisms or with a full prismatic correction, then 
tenotomy of the overacting muscle or muscles must have 
consideration. 

Tenotomy. — As previously stated, tenotomy should never 
be resorted to until every other known means of relief has 
been tried, and even then no hard-and-fast rule can be 
given for the amount of the insufficiency in degrees which 
will prompt such a procedure. Some patients with as 
much as four or six degrees of esophoria may never suffer 
the least annoyance ; and yet other patients with the same 
amount will estimate their sufferings as almost beyond en- 
durance. And the same statement holds good in other 
forms of insufficiency, especially exophoria. The question of 
personal equation, the patient's nervous system, hysteric 
tendencies, etc., must all be considered before undertaking 
a tenotomy that may result in nothing but discourage- 
ment. 

If an operation has been deemed best, then it is for the 
surgeon to decide whether he will divide the tendon of the 
strong muscle or advance the weak muscle, or both. What- 
ever operation or operations are performed, the amount of 
the deviation should be estimated immediately before, as well 
as during and after, the operation. When a simple tenot- 
omy is performed, the eye is usually left open (unband- 



MUSCLES. 195 

aged) so that visual fixation is maintained, and the muscle 
balance tested frequently to see that, by subsequent con- 
traction, the insufficiency does not return. To avoid such a 
misfortune it may be necessary to use prism exercises dur- 
ing the healing process. The writer is not an advocate of 
partial tenotomies. 

Strabismus (arpitpw, " to turn aside ") ; also called heter- 
otropia, " cross-eye " or " squint," or manifest squint. This 
is a condition of the eyes in which the amount of the 
insufficiency is so great that it can not (always) be over- 
come by muscular effort ; and, in fact, inspection often shows 
the manifest condition. Or strabismus may be defined as 
the condition in which the visual axis of one eye is deviated 
from the point of fixation. The eye which has the image 
of the object on its fovea is spoken of as the fixing eye, 
while the other eye is termed the squinting or deviating 
eye. The squinting eye does not always have normal 
visual acuity ; and, in fact, correcting lenses will not 
always produce such a result. 

Varieties of Strabismus. — Convergent, divergent, 
vertical, monolateral, alternating, periodic, concomitant, 
and paralytic. 

Convergent squint (con, "together," and vergere, "to 
incline or approach ") ; also called internal squint (strabis- 
mus convergens), esotropia. This is the condition in which 
the visual axis of one eye is deviated inward, the other 
fixing the object ; or one eye fixing an object, the visual 
axis of the other eye crosses that of the fixing eye closer 
than the object. (See Fig. 163.) This is the most common 
form of squint. Both eyes have some form of hyperopia, 
as a rule, the squinting eye usually being the most ame- 
tropic. The diplopia as a result of this condition is 
homonymous. 



I96 REFRACTION AND HOW TO REFRACT. 

Divergent squint (di, " apart," and vergere, "to in- 
cline ") ; also called external squint (strabismus divergens), 
exotropia. (See Fig. 164.) This is the condition in which 
the direction of the visual axis of one eye is directed out- 
ward, the other eye fixing the object ; or one eye fixing 
an object, the visual axis of the other eye can. cross it only 
by being projected backward. The diverging eye is usually 
myopic. 

Monolateral (one-sided) squint ; also called constant. 
It may be either convergent or divergent, but the squint is 
a constant condition of one eye. 

Alternating Squint. — This is the condition in which 
at different times the right eye fixes and the left eye squints, 
or the left eye fixes and the right eye squints. The vision 
in one eye may be as good as that of its fellow. 

Periodic squint ; also called intermittent. This is the 
condition in which the visual axis of one eye occasionally 
deviates. It may eventually become constant, and is often 
the first indication of a beginning convergent or divergent 
squint. 

Vertical Squint. — This is the condition in which the 
visual axis of one eye is deviated upward. Also called 
hypertropia. 

Concomitant Squint. — This is the condition in which 
the squinting eye has freedom of movement and will follow 
its fellow, and yet one eye deviates (inward or outward) 
because of an inability to " fix." 

Paralytic Squint. — This is the opposite condition from 
concomitant, in which there is a restriction in the move- 
ment of one eye in a certain direction, due to a palsy of 
one or more of the muscles. 

Causes of Squint. — These are many and various. The 
chief causes, however, are : (1) Ametropia, which may pro- 



MUSCLES. 197 

duce a change in the normal relationship between accom- 
modation and convergence ; (2) anatomic anomalies ; (3) 
mechanic anomalies ; and (4) amblyopia. 

I. Ametropia produces a change in the normal relation- 
ship between accommodation and convergence. While it is 
possible for accommodation to take place without conver- 
gence, or convergence without accommodation, yet there is 
an affinity between the two processes which, if materially in- 
terfered with, will produce diplopia and eventually squint. In 
speaking of relative hyperopia, it was shown that the accom- 
modative effort was accompanied by contraction of the inter- 
nal recti muscles (convergence) ; so that in hyperopia of, say, 
four diopters, accommodating for infinity convergence would 
be stimulated to a proportionate degree at the same time ; 
and if accommodating for a near point, the hyperope must 
accommodate and converge just that much more. The 
result is that a person with a hyperopia of any considerable 
amount frequently squints inward in the effort to maintain 
binocular vision. If, now, one eye is more hyperopic than 
the other, the difficulty of adjusting convergence to accom- 
modation is increased. Say that the right eye has 3 
diopters and the left 4 diopters of hyperopia ; then the two 
eyes each exert 6 diopters to fix at 13 inches ; the left eye 
still has 1 diopter of its hyperopia remaining, and with the 
result that the retinal image of that eye is not clear, and 
accommodation is still further taxed, stimulating at the 
same time the internal rectus, so that the left eye deviates 
inward and ultimately remains convergent. This act of 
convergence explains the presence of convergent squint in 
hyperopia, and also shows why the squinting eye usually 
has the higher refractive error. It must not be supposed 
that all hyperopic eyes have a squint, as some of these can 



I98 REFRACTION AND HOW TO REFRACT. 

accommodate without converging in a proportionate de- 
gree, and this is especially so when the amount of the 
hyperopia is the same in both eyes. 

Myopic eyes, in contradistinction to hyperopic eyes, can 
not accommodate beyond their far points, but must con- 
verge. If the myopia is 8 diopters, then these eyes would 
have to converge 8 meter angles to fix an object at that 
distance (5 inches) without any accommodative effort. It 
must also be borne in mind that myopic eyes are long eyes, 
and that to converge 8 meter angles means a great effort 
on the part of the internal recti muscles, and this force can 
not be continued for any length of time without discomfort ; 
the result is, convergence is relaxed, and, one eye remaining 
fixed, the other is turned outward. This is much more 
likely to happen if one eye is more myopic than the other. 
This explains the presence of divergent squint in cases of 
myopia. But it must not be supposed that all myopic eyes 
necessarily have squint, as some of them have roomy orbits, 
strong internal recti muscles, and a short interpupillary 
distance. 

2. Anatomic Anomalies. — This applies especially to the 
breadth of the face (skull) and the size of the eye and 
orbit. The broad face, which naturally gives a long inter- 
pupillary distance, predisposes to greater convergence than 
the narrow face. The long, myopic eye would not have 
the freedom of movement that the short eye possesses in 
the same-sized orbit. 

3. Mechanic Anomalies. — This refers especially to the 
length and strength of the extraocular muscles. Short 
and strong internal recti would predispose to convergent 
squint, whereas strong external recti would develop diver- 
gent squint. 



MUSCLES. 1 99 

4. Amblyopia. — Statistics show that from thirty to 
seventy per cent, of all squinting eyes are amblyopic. The 
cause of the amblyopia may be that the eye was born 
defective in its seeing quality — i. e. y the cones at the fovea, 
the optic nerve, or the visual centers in the brain may be at 
fault. Or if born perfect and having its visual axis deviated 
by one of the many causes above mentioned, it may be- 
come amblyopic from not being used (amblyopia exanop- 
sia). This consideration of cause and effect is most impor- 
tant from a prognostic point of view. 

Among other causes of squint must be mentioned opaci- 
ties of the media, as nebula of the cornea, or any want of 
transparency in the cornea at or near the visual axis, or 
polar or nuclear cataract. Temporary or intermittent 
squint may result from vitreous opacities, or from the rem- 
nant of a hyaloid artery passing in front of the fovea. Parents 
occasionally delude themselves with the idea that the 
child's squint is the result of whooping-cough, measles, 
teething, sucking the thumb, or imitating a companion, 
etc., and are slow to believe that there can be any refrac- 
tive error, forgetting that the supposed causes they men- 
tion may be but coincidences. 

To Estimate the Amount of the Strabismus or 
Squint. — This is not always easy at the beginning of the 
examination, for the reason that the squinting eye has long 
since learned to ignore the false object ; and if the angle of 
the strabismus is large, the surgeon will have to reduce it in 
part with a prism, so that the patient can see the false object ; 
and if this is a point of light, a piece of dark red glass will 
have to be placed in front of the fixing eye. The strength 
of the prism required to bring the two lights together will 
be the prismatic estimate of the deviation. Or the amount 
of the squint may be roughly determined with the strabis- 



200 



REFRACTION AND HOW TO REFRACT. 



mometer. (See Fig. 170.) This is a piece of bone or ivory 
hollowed on one side so as to fit the curve of the eyeball. 
Its edge is graduated in millimeters. This device is held 
gently against the lower lid of the squinting eye, so that 
the zero (o) mark corresponds to the center of the pupil as 
the eye fixes a distant object, the fellow-eye being under 
cover. When the cover is removed, 
the squinting eye again deviates, 
and the amount of the deviation is 
again noted by the position of the 
center of the pupil of the squinting 
eye over the millimeter line on the 
instrument. Each millimeter of 
deviation is supposed to represent 
5 degrees of deviation. This device 
is not reliable, and is not in common 
use. 

A more reliable estimate is ob- 
tained by measuring the deviation 
on the arc of the perimeter. (See 
Fig. 171.) To do this, the patient 
is seated with the squinting eye 
opposite to the fixation point (R) 
and instructed to look at a distant 
object (R) across the room, so that the object, the fixation 
point, and the squinting eye (R) are in line ; this line repre- 
sents the direction which the eye would take normally. 
The observer, taking a lighted candle, places it at the fixa- 
tion point and gradually moves it outward along the inner 
surface of the arc until his own eye, directly back of the 
flame, sees an image of the flame at the center of the pupil 
of the squinting eye. The degree mark on the arc from 
which the flame was pictured represents the amount of the 




Fig. 170. 



MUSCLES. 



201 



deviation or angle of the strabismus ; this angle being 
formed by the visual axis with the direction of the normal 
visual line. The degree mark on the arc is in front of the 




optic axis and not the visual axis, but for purposes of 
approximation they are considered as the same. 

Treatment of Strabismus. — As ametropia is the chief 
factor in the cause of squint, this cause must be promptly 



202 REFRACTION AND HOW TO REFRACT. 

removed by the use of correcting glasses. The correction 
of the ametropia means four essentials : 

1. In young subjects the eyes must be put at rest, and 
kept at rest for two, three, or four weeks, with a reliable 
cycloplegic and dark glasses. Preference is given to 
atropin in each instance, the writer considering it folly to 
use homatropin in such cases. 

2. During the use of the cycloplegic, the lenses which 
correct the ametropia are selected with care and the greatest 
precision, by every known means to this end ; and just 
here is the place of all places to use the retinoscope, as 
most cases of strabismus appear in children, and, too, the 
squinting eye often being amblyopic, can not assist in the 
selection of the glass. 

3. The correcting glasses are ordered in the form of 
spectacles, and are to be worn from the time of rising until 
going to bed. The strength of the glasses should be as 
near the full correction as it is possible to give. 

4. The " drops " are continued for a day or two after 
the glasses have been obtained, and in this way, while the 
drops are still in the eyes, and as their effect slowly wears 
away, the eyes gradually become accustomed to the new or 
natural order of accommodation and convergence. After 
the cycloplegic has entirely disappeared, the patient should 
be carefully restricted in the use of the eyes for near-work 
for several days or weeks. 1 

As hyperopia and astigmatism in combination are gener- 
ally congenital conditions, it therefore follows that conver- 
gent squint appears quite early in life, as soon as the child 
begins to concentrate its vision on near objects. The 
squint, at first periodic or intermittent, finally becomes con- 
stant. Such eyes should be refracted at once, and before 
amblyopia exanopsia can be established. It is interesting 



MUSCLES. 203 

to note that the eyes in many young children begin to fix 
or lose their squint as soon as cycloplegia is established. 
The prognosis is favorable for good vision with glasses 
when this occurs. It will also be observed in other sub- 
jects that while the drops are in the eyes and glasses 
worn constantly, the squint disappears entirely ; but as soon 
as the cycloplegia passes away and near vision is attempted ; 
the squint returns, and vision falls back in the squinting 
eye to almost the same point that it had before the cyclo- 
plegia. This occurs in cases where the amblyopia is becom- 
ing established, or where there is a strong muscle devi- 
ating the eye. If the squint is due to amblyopia exanopsia, 
then the vision may be improved in one of two ways, as 




Fig. 172. 

suggested by Dr. G. M. Gould, " Amblyopiatrics," "Med. 
News," Dec. 31, 1892. One way is to use drops in the 
fixing eye, and thus compel the squinting eye to do the 
seeing ; or the other way is to cover the fixing eye with a 
blank over the glass (see Fig. 172), and have the patient 
practise in this way for one or two hours each day, using 
the squinting eye alone. 

Worth's Amblyoscope or " Fusion Tubes/ ' — To 
cultivate or develop binocular vision Worth has given us an 
instrument which he calls an amblyoscope. (See Fig. 173.) 
This instrument consists of two halves joined by a hinge. 
Each half consists of a short tube joined to a longer one 
at an angle of 120 degrees; at the junction of the tubes is 



204 REFRACTION AND HOW TO REFRACT. 

an oval mirror. A translucent glass object slide is placed 
at the distal end of each tube. At the hinged ends are 
lenses whose focal length equals the distance of the re- 
flected image of the object slide ; in front of these lenses are 
grooves into which additional lenses of the trial case may 
be placed to correct the refractive error of the patient. The 
two halves of the instrument are united by an arc, having a 
long slot at one end and an adjusting screw at the other. 
The object slides can be brought together to suit a 
convergence of 60 degrees, or a divergence of the visual 
axes of 30 degrees. When the adjusting screw is used an 
additional movement of 10 degrees is obtained. 




Fig. 173. — Worth's Amblyoscppe. (Reduced size.) 

At the far end of each tube there is also a square slot 
into each of which may be placed half a pictured object ; 
for instance, a picture of the right side of a man, showing 
his arm and leg extended, may be placed in the left tube, 
and in the right tube is placed a picture of the same size, 
of the left side of the man with his leg and arm similarly 
extended. When the patient looks into the tubes, the sur- 
geon (or the patient) may adjust the tubes until the two 
half pictures unite and form one complete picture. Or the 
picture in one tube may be a picture frame, and in the other 
tube is a picture of an animal or an object, the idea being 



MUSCLES. 205 

to have the patient so fuse the two pictures that the object 
is placed in the frame. There are many different pictures 
accompanying the instrument so as to give variety to the 
daily exercises and thus maintain the patient's interest. 
This instrument is certainly a valuable one and in many 
instances accomplishes its purpose. 

Cases that are cured by correcting the ametropia must 
wear their glasses constantly. Glasses in such cases can 
seldom be abandoned. In young children the squint re- 
turns almost at the instant the glasses are removed. The 
earliest age at which glasses can be prescribed is three 
years or thereabouts, as it would be unreasonable in most 
cases to expect a child to appreciate the glasses as anything 
but a toy before this age. 

The younger the patient when glasses are prescribed, the 
more favorable the prognosis and less likelihood of a ten- 
otomy. The older the patient when glasses are ordered, 
the less the likelihood that glasses will cure the squint and 
the greater probability of a tenotomy being necessary. 
This is explained from the fact that the squint having per- 
sisted for a long time, the muscle which held the eye in the 
deviated position has grown strong and the opposing mus- 
cle weak. 

The correction of squint by glasses applies particularly 
to cases of the concomitant (convergent or divergent) form. 
Vertical squint is seldom cured by correcting glasses alone. 
Prisms will occasionally substitute for an operation. 

Monocular and alternating squint are greatly relieved 
by the correction of the ametropia, and may or may not 
be cured with glasses alone. 

Periodic or intermittent squint, if due to permanent opaci- 
ties in the media, can not, as a rule, be cured by any form 
of treatment. 



206 REFRACTION AND HOW TO REFRACT. 

Paralytic squint is not a part of the subject-matter of this 
work. Cases of concomitant squint are generally amenable 
to operative treatment, whereas cases of paralytic squint are 
not. 

It may be stated as a good rule to follow that no case 
should ever be operated upon until the glasses which cor- 
rect the ametropia have been worn constantly for several 
weeks after all apparent improvement has ceased. If cases 
for operation can be selected, the best age is about puberty, 
when the muscles have reached a fair state of development. 
If the squint is due to an anatomically short muscle, then 
there need not be any great delay in operating after glasses 
have been ordered. 

Whenever a tenotomy has been performed, the eyes 
should again be carefully refracted, as it is a well-estab- 
lished fact that tenotomy often relieves a tension that will 
materially change the radius of corneal curvature ; and 
hence the amount of the astigmatism and the cylinder axis 
will be altered. 

Tenotomy. — For convergent squint, if of moderate 
degree, division of the tendon of the internus of the con- 
verging eye may be sufficient ; but if the squint is consider- 
able, the tendons of both interni may have to be divided. 
Occasionally, it is necessary to divide the internus and 
advance the externus. 

For divergent squint, if of moderate degree, division of 
the tendon of the externus of the diverging eye may be 
sufficient ; but if the squint is considerable, the tendons of 
both externi may have to be divided. Occasionally, it is 
necessary to divide the externus and advance the internus. 

For vertical squint, tenotomy of the stronger superior or 
stronger inferior rectus, or both, may be necessary. 

It is good practice in every instance, before " rushing " 



MUSCLES. 207 

into an operation for squint, to take the field of vision and 
search carefully for a central scotoma, which, if present, 
should put the surgeon on his guard against operative 
interference with the hope of obtaining any result other 
than cosmetic ; and even then there is grave danger that 
the case will soon lapse into the former state of deviation, 
or possibly deviate in the opposite direction. 



CHAPTER VIII. 

CYCLOPLEGICS.— CYCLOPLEGIA.— ASTHEN- 
OPIA.— EXAMINATION OF THE EYES. 

A cycloplegic (from the Greek, xuzkoq, "a circle," — i. e., 
the ciliary ring, — and ntypj, " a stroke ") is a drug which 
will temporarily paralyze the action of the ciliary muscle. 

A mydriatic (from the Greek, iwdpia<ris, "enlargement 
of the pupil ") is a drug which will temporarily dilate the 
pupil. 

Atropin will dilate the pupil and also cause a paralysis 
of the ciliary muscle. Cocain will cause a dilatation of the 
pupil, but will not paralyze the action of the ciliary muscle. 
A cycloplegic is also a mydriatic, but a mydriatic is not 
necessarily a cycloplegic. 

The Uses of a Cycloplegic. — (i) To temporarily sus- 
pend the action of the ciliary muscle, or to put the eye in 
such a state of rest that all accommodative effort is for a 
time suspended while the static refraction is being estimated. 
(2) The retina and choroid are given an opportunity to 
recover from irritation and congestion incident to eye -strain 
(" eye-stretching "). There are many different cycloplegics 
employed for estimating the static refraction, and each has 
particular qualifications for individual cases. Cycloplegics 
may be classed as of three kinds : (1) those the effect of 
which passes away slowly ; (2) those the effect of which 
passes away moderately fast; and (3) those the effect of 
which is very brief. 

The first effect of a cycloplegic is its mydriatic quality, 

208 



CYCLOPLEGICS. 200, 

after which the accommodative effort is suspended. The 
paralysis is not permanent. The following table, from Jack- 
son, shows the length of time paralysis persists and the 
time it takes for the ciliary muscle to fully recover : 

Atropin, effect begins to diminish in 4 days ; complete recovery, 15 days. 

Daturin, " " " 3 " " " 10 

Hyoscyamin, " " " 3 " " " 8 

Duboisin, " " " 2 " " " 8 

Scopolamin, " " " 12 hours. " " 6 

Homatropin, " " " 12 " " " 2 

If a solution of one of the above-mentioned cycloplegics be 
instilled into the conjunctival sac of a healthy eye, it will be 
carried by the blood- and lymph-vessels at the sclerocorneal 
junction into the ciliary muscle and iris, where it acts 
directly upon the nerves and ganglia of these structures, 
and the aqueous humor also receives some portion of the 
drug. If cautiously used, the action will be limited to one 
eye, showing that the drug does not pass through the car- 
diac circulation ; otherwise, the pupil and ciliary muscle of 
the fellow-eye would be similarly affected. 

Some conjunctivas are very sensitive to any of these 
drugs, and develop an inflammation so severe in individual 
instances as to resemble ivy poisoning of the lids. Duboisin 
especially, and hyoscyamin, by absorption, may develop 
hallucinations and even a loss of coordination. 

Any cycloplegic, in fact, when carelessly used, may pro- 
duce very unpleasant symptoms, such as dizziness, dry 
throat, flushed face and body (mistaken for scarlatina), rapid 
pulse, a slight rise of temperature, and delirium. To avoid 
such an annoyance, which is apt to reflect discredit upon 
the physician and upon the profession in general, the patient 
should always be given definite instructions how to use the 
drug in each instance. Stopping the use of the drug and 



2IO REFRACTION AND HOW TO REFRACT. 

applying cold compresses will relieve the conjunctivitis, and 
if constitutional symptoms manifest themselves, a dose of 
paregoric, cooling drinks, a darkened room, and stopping 
the use of the drug will soon restore the patient. 

Form of Prescription. 
Name, Mr. Brown. 

U . Atropin. sulphatis, • . . . gr. j 

Aquae dest, . . . fspj. 

M. Ft. sol. Label, poison drops ! ! 

Sig. — One drop in each eye three times a day, as directed. 

R . Dropper. Dr. 

Date, Tuesday, March 14, 1899. 

The reason for labeling this prescription "poison drops " 
is not to frighten the patient, but to caution him against 
leaving the medicine where children may get hold of it, 
and at the same time to let him understand that it is to be 
used and handled with care. 

Mr. Brown is told to have one drop put in each eye 
three times a day, after meals, and to report at the office on 
Thursday (the prescription is given on Tuesday in this case). 
The reason for using the drug for this length of time is to 
insure complete paralysis, and also to give the eyes a physi- 
ologic rest. In having these " drops" put in the eyes, the 
patient should tip his head backward and turn his eyes down- 
ward, and as the upper lid is drawn up, one drop (from the 
dropper) is placed (not dropped) on the sclera at the upper and 
outer part. After the drops are placed in the eyes, as far away 
from the puncta lachrymalia as possible, the patient holds 
the canaliculi closed by gently pressing with the ends of the 
index fingers on the sides of the nose at the inner canthi 
for a minute or two. If more than one drop enters the eye, 
it will run over on to the cheek, and should be wiped off. 
With children, these instructions are not so easy of exe- 



CYCLOPLEGICS. 2 I I 

cution, and the writer has seen a few such clinical subjects 
flushed and delirious from gross carelessness on the part of 
parents in dropping the medicine into the inner canthi, 
where it soon passed into the nose, or else the drug is 
allowed to flow over the cheek and into the child's open 
mouth. Ordinarily, there need never be any discomfort 
from the use of these drugs beyond a slight dryness of the 
fauces. 

Caution. — Cycloplegics should never be used when 
there is the least suspicion of glaucoma in one or both 
eyes. Cycloplegics should not be used in the eyes 
of nursing women ; such patients are peculiarly suscep- 
tible to the action of these drugs, and the mammary 
secretion may thereby be diminished in amount. After the 
age of forty-five or fifty years, or in the condition known 
as presbyopia, it is seldom necessary to use a cycloplegic. 
If a cycloplegic is necessary in presbyopia, one of the 
weaker drugs is generally employed. 

In the selection of a cycloplegic the surgeon must be 
guided by the patient's occupation, age, the character of 
the eyes, and the refraction. From the foregoing table it 
will be seen that atropin and daturin are slow in passing 
from the eye, making their employment on this account 
very objectionable in many instances. The accommodation 
returns sooner after the use of hyoscyamin and duboisin 
than from atropin, but not so promptly as from scopolamin 
and homatropin. The effect of the latter is very brief. A 
patient who might lose his business position if he remained 
away from work for more than a week could not afford to 
have atropin or daturin used in his eyes, whereas a school 
child might accept atropin as a luxury. The man of busi- 
ness, the cashier in a bank, the storekeeper, and others 
must, in many instances, have their eyes refracted in at 



212 REFRACTION AND HOW TO REFRACT. 

least two days ; and this latter time means, of course, the 
use of homatropin. The nearer the age to forty years, the 
less need for one of the stronger cycloplegics, as the power 
of accommodation has markedly diminished at this period 
of life, so that hyoscyamin or scopolamin will answer every 
purpose. After thirty-five years homatropin can, as a rule, 
be relied upon as a cycloplegic. 

In hyperopic eyes of young subjects it is useless to em- 
ploy homatropin, as the active ciliary muscle requires a 
strongly acting cycloplegic to stay the accommodative 
power. In myopic eyes one of the stronger cycloplegics 
may be used to advantage, for the following reasons : 
Myopic eyes have large pupils, as a rule, and do not mind 
the mydriasis ; myopic eyes are often in a state of irritation, 
and the drug gives them a much-needed rest ; the myope's 
distant vision is not disturbed by the cycloplegic, as in the 
case of the hyperope. 

Whenever a cycloplegic is prescribed, the patient should 
be ordered a pair of smoked-glass spectacles to wear dur- 
ing the mydriasis. Of the two forms of smoked glasses,— 
coquilles and plane, — the latter should always be preferred, 
as they are without any refractive quality, whereas coquilles 
have some form of refraction that may act very injuriously. 
Another reason for ordering the plane glass is that the 
patient will often wish to wear them with his prescription 
glasses, which he could not do so well if they were coquilles. 
Dark glasses are of four shades of " London smoked " — A 
B, C, and D, A being the lightest shade and D the darkest. 
The prescription would be : 

For Mr. Brown : 

R . One pair plane London smoked " D." 

SlG. — For temporary use. Dr. 

March 14, 1899. 



CYCLOPLEGICS. 2 I 3 

The cycloplegics above mentioned for purposes of re- 



fraction are ordered in the following strengths : 



Atropin. sulphatis, gr. j'to aq. dest., 

Duboisin. sulphatis, gr. ss " " " 

Hyoscyamin. sulphatis, gr. ss " " " 

Daturin. sulphatis, gr. ss " " " 

Scopolamin. hydrochlor. , . . . . gr. j " " " 



f 3 iss. 



All these, except scopolamin, are ordered to be used 
three times a day, preferably after meals ; but scopolamin 
being a very powerful drug, the surgeon should place it in 
the patient's eyes himself in the office, and not give a pre- 
scription for it. Only two drops are necessary, and are 
instilled a half-hour apart, the static refraction being esti- 
mated one hour after using the second drop. 

How to Use Homatropin. — This drug is expensive, and 
it is never necessaiy to prescribe more than one grain for 
any one patient. Personally, the writer has found the fol- 
lowing most satisfactory, though the strength of the homa- 
tropin may be increased if desired : 

For Miss Robinson : 

B; . Homatropin hydrobromate, gr. j 

Aq. dest., TT^ xl. 

M. Ft. sol. Label, poison drops ! ! 

SlG. — One drop in each eye, as directed. 

R. Dropper. Dr. 

March 14, 1899. 

One drop of this solution instilled into a healthy eye will 
produce mydriasis in a few minutes, but its action on the 
ciliary muscle is so trifling that the near point will be but 
slightly changed. It is thus shown that this drug is a 
decided mydriatic, and only becomes a cycloplegic under 
definite usage. 

To produce cycloplegia with homatropin, the patient is 



214 REFRACTION AND HOW TO REFRACT. 

given the above prescription and told to use it as fol- 
lows : 

To place one drop in each eye at bedtime the first night. 
This one drop dilates the pupil and establishes a change in 
the circulation of the blood-supply to the iris and ciliary 
body — a very important matter for the patient's comfort, and 
at the same time preventing a tendency to spasm of the 
ciliary muscle. The next morning one drop is to be placed 
in the eye every hour, from the time of rising until leaving 
home to go to the surgeon's office. At the office one drop 
is placed in each eye about every five minutes, until six 
drops have been used ; then, after waiting half an hour 
(for the cycloplegic effect, which will last for one hour), the 
refraction is carefully estimated. After a short interval the 
cycloplegic effect will begin to rapidly disappear, so that the 
patient will be able to read within forty-eight hours' time 
with his correcting glasses. 

Occasionally, a busy patient will insist upon having his 
eyes refracted during his first visit, and can not take time 
to use the drops in the manner above suggested. The 
surgeon must, therefore, start and use the drops in his 
office. This is forcing the ciliary muscle into a state of 
paralysis that does not always give ultimately satisfactory 
results. "Forcing" homatropin into an eye in this way 
will always produce a " bloot-shot " eye (hyperemia of the 
conjunctiva, etc.) that does not improve a patient's appear- 
ance ; and it often produces severe neuralgic headache that 
may result in nausea or vomiting in occasional instances. 
Furthermore, it is possible, with a drug like homatropin, if 
not properly used, to have some of the sphincter-fibers 
become paralyzed while others may remain free to act. In 
this way a spasm of the ciliary muscle may be produced 
that will give a false astigmatism. Personally, the writer 



CYCLOPLEGICS. 21 5 

is not partial to this method of forcing the ciliary muscle 
into repose. 

To somewhat obviate the "blood-shot" condition of the 
eye, and also to assist the action of the "forcing" process, 
one drop of a two or four per cent, solution of cocain 
may be instilled while the homatropin is being used. This 
also diminishes the danger of spasm. But cocain is objec- 
tionable in that it will, in some cases, "haze" the cornea. 
The retinoscope will show this, and the patient will state 
that, while he can see the letters on the test-card, yet they 
have a "mist" over them. Instead of using the hom- 
atropin alone, a small amount of cocain may be added to the 
solution for the purpose mentioned. Or, homatropin may 
be combined with cocain and chlorid of sodium in the form 
of a disc, and one of these, placed in the conjunctival sac, is 
allowed to dissolve, and in this way paralyze the accommo- 
dation. Or, homatropin may be used in a solution of dis- 
tilled castor oil. It is claimed that when the drug is used 
in this form, it remains in contact with the tissues and acts 
more energetically. 

Homatropin as a cycloplegic should be held in reserve 
for individual cases, and not used as a routine practice. It 
is a good, reliable paralyzer of the accommodation in many 
eyes at the age of thirty-five, or thereabouts ; but in a young 
hyperopic eye it is a waste of time to attempt successful 
paralysis with it, and the danger of producing a false astig- 
matism should certainly deprecate its use in these cases. 
Another very serious objection to its use is that before the 
eyes can become accustomed to the prescription glasses, 
the ciliary muscle recovers and begins to accommodate, 
with the result that the patient says he can see better at a 
distance without his glasses than he can with them, and 
has no small amount of mistrust of the surgeon's ability, 



2l6 REFRACTION AND HOW TO REFRACT. 

as he will have to wear his glasses a long time before his 
ciliary muscle will relax its accustomed accommodative 
efforts. This is not nearly so likely to occur if one of the 
slowly acting cycloplegics is used. 

The method of refracting with one of the slowly acting 
cycloplegics, and then endeavoring to counteract the effect 
with a solution of eserin, is not recommended. Temporarily, 
eserin may overcome the cycloplegic ; but as its action is 
only transitory, the paralysis reasserts itself and will not 
disappear until the specified time. 

Refracting one eye at a time with a cycloplegic while the 
patient pursues his occupation with the other eye is not a 
method to be considered. This means a great amount 
of discomfort, headaches, eye-strain, and even diplopia at 
times, during so prolonged a treatment. 

If a hyperopic patient must occasionally use his eyes for 
near work while he has drops in them, a pair of +3 or 
-j-4 spheres may be given for temporary use. 

Cycloplegia. 

Cycloplegia is a paralysis or paresis of the ciliary muscle. 
This condition may be monocular or binocular ; it may be 
partial or complete. Mydriasis may or may not accompany 
the cycloplegia, though the two conditions usually occur 
together ; and when they both exist, the paresis is spoken 
of as ophthalmoplegia interna. The ciliary muscle and 
sphincter of the iris are controlled by branches from the 
third nerve ; but these branches are from independent cen- 
ters ; the fibers going to the ciliary muscle arise beneath the 
floor of the third ventricle, in front of the fibers which go to 
control the sphincter of the iris. 

Causes. — Temporary paralysis of the ciliary muscle and 
iris, as already stated, will result from the external or internal 



CYCLOPLEGI A. 2 1 7 

administration of a cycloplegic. It is interesting, in many 
cases, to find the cause and relieve the patient's anxiety 
when the paresis is due to one of the cycloplegics. Aside 
from the use of eye-drops, the question of external 
medication (liniments, ointments, and plasters) should be 
inquired into, as also whether rectal or vaginal supposi- 
tories containing a cycloplegic have been used. 

Other causes of this form of paralysis are tonsillitis, quinsy, 
diphtheria, Bright's disease, rheumatism, gout, exhausting 
diseases, blows upon the eye, etc. Other and more serious 
causes, as controlling a guarded prognosis, are intracranial 
hemorrhage, meningitis, syphilis, brain tumor, etc. In 
some instances the cause can not be definitely ascertained. 

Symptoms and Diagnosis. — Photophobia, dilatation of 
the pupil, and loss of accommodative power consistent with 
the optic condition of the eye. 

A myopic eye retains its vision at the far point only ; an 
emmetropic eye or a hyperopic eye wearing correcting 
glasses has good distant vision and absence of a near 
point ; an uncorrected hyperopic eye has poor distant and 
near vision. 

Prognosis. — This depends upon the cause. 

Treatment. — This must be symptomatic and expectant, 
with a removal of the exciting cause, if possible. As many 
cases of cycloplegia are the result of, or follow, an attack of 
diphtheria, or a disease which has reduced the system below 
par, tonics, fresh air, etc., must be ordered. When brought 
on by syphilis, mercury and iodid of potash must be 
prescribed. Dark glasses for the photophobia should 
always be ordered, and lenses for near-work may be worn 
as a temporary expedient. The use of eserin locally will 
occasionally do good work, but is not advised for constant 
use or for every case. Faradism may be used if the cyclo- 
19 



2l8 REFRACTION AND HOW TO REFRACT. 

plegia is very persistent, but the best results may be ex- 
pected from systemic treatment. The use of strychnin or 
nux vomica are recommended in certain instances. 



Cramp of the Ciliary Muscle. 

Cramp of the ciliary muscle is the opposite condition to 
that of cycloplegia, just described. Ciliary cramp may 
occur in one or both eyes, usually in both ; it may occur 
in any form of ametropia or in emmetropia. Ciliary 
^ramp is of two kinds — clonic and tonic. 

Clonic cramp is an occasional and temporary condition 
which comes on while the eyes are in use, and passes away 
soon after the eyes have had an opportunity to rest, and 
may not occur again for several days. 

Tonic cramp, also called " spasm " of the accommodation, 
is a permanent condition as compared with the clonic form, 
and occurs whenever the eyes are used for distant or near 
vision. The patient can not use his eyes for any length of 
time, or with any considerable concentration, without suffer- 
ing as a consequence. 

Causes. — Clonic cramp may occur as one of the early 
symptoms of presbyopia. Ametropia is a very common 
cause, and especially in cases of low amounts of hyperopia 
or myopia. Emmetropia, or eyes made emmetropic with 
glasses, may develop clonic or even tonic cramp if the eyes 
are used to excess or in a bad light. Such cases have been 
called " hyperesthesia of the retina." Tonic cramp may 
develop from the same causes which bring on the clonic 
form, and is usually seen among young hyperopic children, 
or the "pseudo-myope " already described. It also occurs 
occasionally in hysteric patients or those recovering from 
some severe or long illness. The writer has seen this form 



CRAMP OF THE CILIARY MUSCLE. 2IO, 

of cramp precede or antedate by several weeks a collapse 
of the nervous system — i. e., nervous prostration. 

Symptoms. — Naturally, ciliary cramp means ocular 
pains and headaches. Opera headache, " train headache," 
" shopper's headache," " bargain-counter headache," etc., 
are some of the many names given to cramp of the ciliary 
muscle, and are, no doubt, the result of accommodative effort 
in a bright light or watching moving objects, these symp- 
toms being a part of the history of accommodative astheno- 
pia (already described) and accompanying insufficiency of the 
muscles. Symptoms of myopia are very evident during the 
cramp. In the tonic cramp the ocular pains and headache 
may be so excruciating in individual cases as to make the 
family physician and patient dread cerebral disease until the 
immediate cause is found out. 

Treatment. — As the cause is usually one of ametropia, 
this must be corrected by the careful selection of glasses 
while the eyes are undergoing a prolonged rest with a 
cycloplegic and dark glasses. Later on the patient must be 
cautioned against any overuse of the eyes. The general 
health should have any necessary attention. Sedatives, 
alteratives, and tonics have their place in individual cases. 
Reflex causes must be looked for and, as far as possible, re- 
moved. Insufficiencies should always be carefully searched 
for, and frequently prism exercises to develop the strength 
of the weak muscles may give marvelous results. Un- 
fortunately, there are occasional instances of tonic cramp 
that persist in spite of any treatment, and such cases obtain 
relief only when presbyopia definitely asserts itself. 

Asthenopia (from the Greek, d priv. ; ffSivos, u strength "; 
eu^, " eye " ) means a weakness or fatigue of the eye, applying 
especially to the retina, the ciliary muscle, the extra-ocular 
muscles, or a general weakness of any one or two or all 



220 REFRACTION AND HOW TO REFRACT. 

of these structures in one and the same eye. Asthenopia 
is a disease, and is often spoken of as " weak sight," " eye- 
strain," or "eye-stretching." 

Varieties. — For purposes of study, differential diagnosis, 
and treatment, asthenopia, or eye -strain, has been divided 
into the following varieties : Retinal, muscular, accommo- 
dative, and asthenopia due to a combination of any two or 
all three varieties. 

Retinal Asthenopia. — This is the rarest form of asthen- 
opia, and usually occurs in females. It is brought about 
by overuse of the eyes in too dim or too bright a light, 
and may result from a too prolonged use of the eyes at any 
kind of work or in any kind of light. It may result from 
exposure to the sun's rays, to electric lights, or to light- 
ning, or by reflection from bright objects, such as snow, 
etc. Retinal asthenopia may occur as a symptom of hys- 
teria, or in a patient whose nervous system is peculiarly 
susceptible to vibrations, sounds, and lights ; in a patient 
whose nervous system is an uncertain quantity. Such pa- 
tients are very unsatisfactory to treat or even to examine ; 
they often imagine that the reflected light from the ophthal- 
moscope or retinoscope is "very hot," etc. 

Symptoms. — The chief symptom is a dread of light 
(photophobia), or photophobia and lacrimation together. 

Treatment. — The first thing to do is to remove the 
cause, if this can be found ; otherwise the treatment should 
be very conservative. Ametropia must be corrected and 
the eyes be given some regular work ; in other words, it is 
not good practice to restrict all use of the eyes. The treat- 
ment with "tinted glasses," made so much of by the char- 
latan to " gull " the innocent public, should not be ordered, 
as the patient grows accustomed to them and they event- 
ually become an absolute necessity on all occasions. Care- 



CRAMP OF THE CILIARY MUSCLE. 221 

ful attention to the general health is certainly indicated ; 
tonics, out-door sports, etc., should be prescribed in 
individual cases. The shade of the trees is to be recom- 
mended in preference to the seashore and bright reflection 
from the sand and water. 

Muscular Asthenopia. — This is due to weakness or 
fatigue of one or more of the extra-ocular muscles, most 
frequently the interni (exophoria). Muscular asthenopia 
of the exophoric kind is the result, as a rule, of a want of 
power to maintain convergence. The symptoms are in 
keeping with a cramp followed by a relaxation of converg- 
ing power. Ocular pains, eyeballs tender to the touch (per- 
chance the internal recti themselves become sore to the 
touch or feel sore on movement of the eyes), and in some 
cases the conjunctiva and subconjunctival tissues overly- 
ing the muscles become hyperemic during or after the use 
of the eyes, simulating rheumatism of these structures. In 
other cases dim vision and diplopia will be occasional mani- 
festations. Patients with muscular asthenopia occasionally 
find that they can continue at near work by using one eye, 
but this does not occur very often. 

Treatment. — This resolves itself into the correction of 
the ametropia, exercise of the weak muscles, etc. (See 
chapter on Muscles.) 

Accommodative Asthenopia. — This is by far the most 
common form of asthenopia, and is due to fatigue of the 
ciliary muscle ; it is, therefore, to be expected in hyperopic 
eyes. It is caused in various ways : from overuse of the 
eyes in too bright or too dim a light, or from using the eyes 
for too long a time in any kind of a light. The best pair 
of eyes, if overtaxed, may suffer from accommodative 
asthenopia, even when wearing the ametropic correction. 
Or accommodative asthenopia may result from a weakness 



222 REFRACTION AND HOW TO REFRACT. 

of the ciliary muscle as a part of the general condition of the 
whole body, and this may come on after or during some long 
illness, such as typhoid fever. Accommodative asthenopia 
is often present in the early months of presbyopia. 

Symptoms. — The principal symptom is headache — fron- 
tal, frontotemporal, or fronto-occipital ; or this pain or dis- 
comfort may extend into the neck or between the shoulders. 
The headache develops during the use of the eyes, and 
grows worse if the effort is prolonged, and usually ceases 
after the eyes are rested. See chapter on Hyperopia and 
Myopia. 

Treatment. — When glasses are necessary, they should 
be ordered by the static refraction. The general health of 
the patient should receive careful attention. An out-of- 
door life will often be necessary, and in certain cases the 
time for using the eyes at any near-work will have to be 
very much restricted. 

Accommodative with Muscular Asthenopia. — This 
variety of asthenopia embraces the two forms just men- 
tioned, and its description and treatment are included in both. 

Reflexes Due to Eye-strain. — Among the symptoms 
of the various forms of asthenopia described on the previous 
pages, the writer has avoided any decided reference to reflex 
symptoms, preferring to speak of these reflexes in a general 
way under one heading. Many patients who suffer from 
headaches, ocular pains, etc., during the use of their eyes, 
also very frequently suffer from constipation, indigestion, 
heartburn, nausea, or even vomiting. Other patients 
may have nervous attacks, a fear of some impending 
calamity, or they are irritable or despondent ; they may 
suffer from insomnia, or, if they sleep, it is not a restful 
sleep. Others may have epileptic attacks, nervous twitch- 
ings, etc. To just what extent eye -strain is responsible for 



EXAMINATION OF THE EYES. 2 23 

these and many other reflexes the writer is not prepared 
to say, though every ophthalmologist has certainly seen 
some cases of accommodative and muscular asthenopia with 
gastric symptoms, or nervous symptoms, or epileptic 
attacks, or irritable tempers, or insomnia, or enuresis, etc., 
in which these reflex symptoms entirely disappeared after 
the eye-strain was properly treated. 

Examination of the Eyes. 
A systematic method should be pursued in the examina- 
tion of the eyes, and the results recorded in a book or on 
a card prepared for that purpose. The student should be 
a careful observer, and also be able to question the patient 
intelligently for short and definite answers. The following 
is an excellent method of making records, but there is no 
arbitrary rule, and in this respect each surgeon may follow 
his own desires : 

Date, 

Name, 

Residence, 

Occupation, 

Age, Sex, Diagnosis, 



Accommodation. Astigmatism. Muscles. 

O. D. V., p. p. 

O. S. V., p. p. 

History, 

S. P. [status proesens, " present condition "). Inspection, 

Ophthalmometer, O. D O. S 

Ophthalmoscopic examination, O. D O. S 

Manifest refraction. Fields. Color sense. R . 



224 REFRACTION AND HOW TO REFRACT, 

The above record is filled out as the examination pro- 
ceeds, but it is not always advisable to follow the exami- 
nation in the order given ; on the contrary, it is better, after 
getting the patient's name and address, to ask certain other 
questions which may appear in keeping with an individual 
case. 

1. Occupation. — This is a very important question, as 
bearing directly upon the amount and character of work 
done by the eyes ; for example, writing, reading, sewing, 
music, engraving, weaving, drafting, surveying, painting, 
typewriting, typesetting, sorting colors, etc. 

2. Age. — This is of the utmost importance in comparing 
the range of accommodation (near point) with the emme- 
tropic condition. Knowing the patient's age and near point 
will often give a diagnosis of the character of the refraction. 

3. The name tells the sex, but the question really is 
whether the patient is married, single, widow, or widower. 
If a young married woman, whether she is nursing a young 
child. 

4. History. — Under this heading the questions should 
bear directly upon the eyes. " In what way do the eyes 
cause trouble?" The usual answer to this question is 
"headache" To get a complete history of the headache, 
and be able to differentiate it from headache due to other 
causes, the succeeding questions seem appropriate : 

What part of the head aches ? Is it frontal, occipital, 
temporal, interocular, vertex, or all over the head ? 

When does the headache come on — during or after 
the use of the eyes ? Does it cease after resting the eyes ? 
Is the headache worse when using the eyes by artificial 
light ? Is the headache constant ? Is it periodic ? Is it 
worse at a certain hour of the day ? Is the headache pres- 
ent when first waking in the morning ? Does the head ache 



EXAMINATION OF THE EYES. 22 5 

during or after attending a place of public amusement or 
when shopping ? If a female, is the headache only monthly ? 

The ophthalmologist must not think because a patient has 
a headache that it is surely and always due to the eyes, and 
that glasses are going to cure it. It is for the ophthalmolo- 
gist to find out just what part the eyes take in causing the 
patient's discomfort, and not always expect to cure with 
glasses headaches that have no direct relation to the eyes. 

One of the most common causes of headache which 
may be mistaken for ocular headache is the "brow ache" 
due to malaria, but a history of previous malarial attacks, 
chills and fever, a residence in a malarious district, and the 
fact that it is periodic in character, should certainly give a 
clear differential diagnosis. 

Other patients may not consult the ophthalmologist on 
account of headache, but for a pain in or back of the eyes, 
or back part of the head, or between the shoulders, which 
comes on after any effort of vision. Others may complain 
of a feeling of sand in the eyes, or a burning in the lids, or 
a smarting or itching in the lid margins, or excessive lacri- 
mation, or a feeling of drowsiness as soon as the eyes are 
used for any length of time, or a feeling as if the eyelids 
would stick to the eyeballs. 

The patient's seeing qualities may develop the history of 
poor distant vision and good near vision, or vice versa ; this 
should be inquired into very carefully, and it may be well 
to ask about other members of the family, if they have the 
same condition. Or a history of the vision gradually fail- 
ing or of a sudden loss of sight may be obtained, and pres- 
byopic symptoms should be referred to, if the patient is 
over forty years of age. 

If the patient wears glasses, it is well to inquire whether 
they were ordered by an ophthalmologist or if they are 



226 REFRACTION AND HOW TO REFRACT. 

the patient's own selection. In the former instance a record 
should be made of the character and strength of the lenses, 
and whether the lenses were ordered with or without 
"drops" in the eyes; and if "drops" were used, if the 
effect lasted for two days or longer (slowly or quickly 
acting cycloplegic). . Ask how long the glasses have been 
worn, and if the same symptoms are present that existed 
when the lenses were previously ordered. 

Having made a note of the patient's history, it is next in 
order to study the present condition (status prcesens) : 

i . Breadth of face, its symmetry or asymmetry ; inter- 
pupillary distance. 

2. The eyelids, whether swollen, discolored, or having 
red margins. 

3. The eyelashes (cilia), whether regular, irregular, or 
absent. If there are chalazia, styes (hordeola), inflammation, 
moist or dry secretion at the roots of the cilia (blepharitis). 

4. Inspect the inner surface of the lids and ocular con- 
junctiva for inflammation or growths. 

5. Inspect the lacrimal apparatus in all its parts. 

6. Inspect the cornea for its polish, transparency, and 
regularity. 

7. Depth of the anterior chamber. 

8. Iris, its color and mobility. 

9. Pupil, its size, shape, and position. 

10. Color of reflex from the pupillary area. 

1 1. Palpate to measure the intraocular tension. 

12. Use the cover test at 13 inches for any muscular 
anomaly. 

Following this record of the history and present condi- 
tion, the distant vision and near point are taken for each 
eye, one or more tests for astigmatism are made, the mus- 
cles* are tested for distance (six meters), and the ophthal- 



EXAMINATION OF THE EYES. 227 

mometric measure of corneal curvature may be recorded. 
Finally, and most important of all, the ophthalmoscopic 
examination is made, and the cornea, aqueous, lens cap- 
sule, lens, vitreous, nerve (shape, size, color, cupping, and 
vessels), conus, macular region, etc., and periphery of the 
eye-ground are studied. 

Lastly, fields and color sense, dynamic or manifest re- 
fraction. 



CHAPTER IX. 

HOW TO REFRACT. 

General Considerations. — Before placing lenses in front 
of an eye, the surgeon should be acquainted with at least 
five important facts : 

1. The Patient's Age. — This tells at once, from the 
table on page 70 (which the surgeon should commit to 
memory), what the near point will be if the eyes are emme- 
tropic or standard. 

2. The Near Point. — This will usually indicate hyper- 
opia if beyond, and myopia if closer than, the emmetropic 
near point for the age. 

3. The Distant Vision in Each Eye. — If very defective, 
or if less than — and near point closer than the age calls 
for, myopia is indicated. , Good distant vision and near 
point removed indicate hyperopia. 

4. The distant vision, if recorded with question 
marks, usually indicates astigmatism. 

5. The Results of Testing with the Astigmatic 
Chart. — Darkest lines from XII to VI, or I to VII, or XI 
to V, indicate astigmatism (myopic) with the rule ; or 
darkest lines from IX to III, or VIII to II, or X to IV, 
indicate astigmatism (hyperopic) with the rule. 

It is well to remember that about four patients out of 
five have hyperopia, or one patient in five has myopia, and 
the minus sphere selected almost invariably requires a 
cylinder in combination. Remember, also, that astigma- 
tism is usually with the rule and symmetric, and that plus 

228 



HOW TO REFRACT. 229 

cylinders are generally selected at axis 90 or within 45 
degrees either side of 90, and minus cylinders are gen- 
erally selected at axis 180 or within 45 degrees either side 
of 180. 

The Placing of Trial-lenses. — 1. These should always 
be placed as close as possible to the eyes without interfer- 
ing with the lashes ; and to accomplish this, the trial-frame 
should be easy of adjustment. 

2. The center of the trial-lens must be opposite to the 
center of the pupil. 

3. If the distant vision is very defective, — — , — , ~^., 
15? or ^-, — a strong lens of 2 or 3 D. will often be re- 
quired; whereas, if the vision is ^g or ^-, a weaker lens 
would be called for. 

4. When a spheric lens placed before an eye improves the 
vision, it should not be changed for another unless the vision 
is made better by having its strength increased or dimin- 
ished by placing in front of it another sphere (plus or 
minus) of less strength. For instance, if a +2 sph. has 
been placed before the eye and the vision is improved from 

— to r^y^ ; this -f-2 sph. should not be changed until a 

— 0.50 or — 0.50 sph. has been held in front of it and the 
patient states whether he can read more with it or less without 
it. When a vision of— is approximated, then its accuracy 
must be determined by placing first a +0.25 and then a 
— 0.25 in front of the correction, so as to learn from the 
patient which one, if either, of these lenses improves the 
vision. Or if the correcting lenses selected are weak ones, 
then 0.12, plus and minus, may be used in place of the 
0.25. 

5. Spheric lenses should always be tried before using 
cylinders, and the vision brought as low as possible with 



23O REFRACTION AND HOW TO REFRACT. 

a sphere before combining a cylinder, and, in fact, after 
the vision has been improved as much as possible with 
a sphere, the pointed line-test for astigmatism may be 
brought into use, as very often low errors of astigmatism 
are not recognized until this point in the refraction has 
been reached. Advocates of the ophthalmometer place 
the cylinder before the patient's eye and then add the 
spheric correction. The writer is not partial to this method 
or way of refracting. 

6. When a patient miscalls one or more letters in a cer- 
tain line, the surgeon must not hurry on until these are 
corrected by the patient with a suitable glass, and in 
this way the refraction is gradually worked out until the 
vision is brought to the greatest acuity possible. It is 
never wise to stop with a vision of — , as we are often able 
to get a visual acuity of — -, or occasionally — . 

7. Cylinders. — When a plus cylinder is employed, it is 
placed with its axis at 90, and then slowly revolved (if nec- 
essary) to an axis where the patient says he can see better. 
A minus cylinder is placed at axis 1 80, and revolved in the 
same manner. The rule (4) for changing spheres also ap- 
plies to cylinders — i. e., to increase or decrease the strength 
of the cylinder by placing in front of it a plus or minus 
cylinder of less strength at the same axis. 

8. Axis of th? Cylinder. — -When a patient is not sure 
about an exact axis, though he is sure that the cylinder 
improves the vision, then the surgeon may employ a sphere 
of the strength of the sphere and cylinder combined, and 
use a cylinder of the same strength as before, but with 
opposite sign and at about the opposite axis. For example, 
with -j-2.25 sph. O +0.75 cyl., the patient is not sure if 
the vision is best with the axis at 35, 40, or 45, the sur- 
geon must then use a + 3-QO sph. O — 0.75 cyl, when the 



HOW TO REFRACT. 23 I 

exact axis (at right angles) will usually be selected without 
any hesitancy or doubt. 

9. Proving the Correction. — All tests at the trial-case, 
when a cycloplegic is used, should be confirmed with the 
retinoscope. 

10. Crossed Cylinders. — This is a trial-lens that has 
one meridian minus and the opposite meridian plus. They 
are made of any strength, but for general use the 0.25 cyl- 
inders are employed — i. e., — 0.25 sph. O -f o. 50 cyl. The 
purpose of the crossed cylinders is to increase the refrac- 
tion in one and diminish it in the opposite meridian. For 
example: if -j-2.00 sph. O +1.00 cyl. axis 90 gives a 
vision of rr^, and the crossed cylinder lens is placed in 
front of this combination with — 0.25 at axis 180, and the 
— 0.25 at axis 90, and the vision comes down to —-, it 
shows that the vertical meridian was 0.25 too strong, and 
the horizontal 0.25 too weak, and the result would be 
-j-i-75 sph. O +1.50 cyl. axis 90 degrees. Or, if — 3.00 
sph. has brought the vision to — -, and the crossed cylinder 
lens is placed before it and rotated to axis 1 5 for the 
minus cylinder and axis 105 for the plus cylinder, and the 
vision comes to — , the result would be — 2.75 sph. 
O — 0.50 cyl. axis 15. 

Methods of Estimating Refraction. — To determine 
the refraction of an eye it may or may not (as in presby- 
opia) be necessary to employ a cycloplegic. When the 
refraction is estimated without a cycloplegic, it is spoken of 
as manifest or dynamic (Gr., duvafitq, "power") refraction. 
When a cycloplegic is used, the refractive estimate is spoken 
of as static (Gr. ararlxos, from, lerdvat, " to stand at rest "). In 
one instance the ciliary muscle is permitted to act, and in 
the other it is at rest. A third method is to obtain the 
static refraction and then to estimate the strength of the 



232 REFRACTION AND HOW TO REFRACT. 

glasses to be prescribed after the effect of the cycloplegic 
has passed out of the eyes ; this is spoken of as post- 
cycloplegic refraction. Eyes for refraction are divided into 
two general classes, according to the age of the patient. In 
those under forty-five years of age a cycloplegic is usually 
employed, but after this age a cycloplegic is often dispensed 
with. (See Presbyopia.) 

Fogging Method. — This method simulates the static or 
cycloplegic method, as the ciliary muscle is in great part 
(if not entirely) placed artificially at rest by having in front 
of the eye under examination a plus sphere of sufficient 
strength to more than overcome any ciliary muscle power that 
the eye might otherwise use when looking at a distance of six 
meters. The "fogging" method is so called because dis- 
tant vision is made obscure or "foggy." The eye under 
examination with this strong plus sphere in front of it is, to 
all intents and purposes, — for the time being, at least, — 
myopic. This fact should be carefully borne in mind, as the 
method to pursue in estimating the refractive error is to 
proceed the same as in any regular case of myopic refrac- 
tion. Therefore, this method is only of service in estimat- 
ing the refraction of eyes that have some form of hyperopia 
or simple myopic astigmatism, and is not of service in 
myopia or compound myopia. 

How to Proceed in Hyperopia. — Having estimated 
with the ophthalmoscope that the eye is hyperopic about 
2.50 D., then place a plus 4 D. sphere before the eye and 
have the patient look at the card of test letters at a distance 
of six meters. This has the immediate effect of making 
distant vision very " foggy," but by waiting a few seconds 
this foggy vision will clear slightly and the ciliary muscle 
will relax a part, if not all of its effort to accommodate. 
Then proceed as if refracting a myopic eye by placing a 



HOW TO REFRACT. 233 

— 0.50 sphere in front of the -(-4 D. sphere, and gradually 
increase the strength of the minus sphere until the patient 
reads ^\. If the minus sphere is 1.25 D., then the amount 
of the hyperopia will be -\~ 2.75 D., which is the difference 
between the +4 D. and the — 1.25 D. 

How to Proceed in Simple Hyperopic Astigmatism. — 
Having estimated with the ophthalmoscope or retinoscope 
or ophthalmometer or any of the various ways that the eye 
has hyperopic astigmatism of about 2.50 D., proceed as in 
hyperopia by making the eye myopic by the addition of a 

— 4 D. sphere. Have the patient look first at the card of 
test letters and then at an astigmatic clock-dial next to the 
letters. Proceed by adding minus spheres as in the previ- 
ous case, and as soon as the patient recognizes one series 
of lines on the clock-dial as much darker than those at 
right angles to these dark lines, then place minus cylinders 
in position with their axes at right angles to the darkest 
lines, and as soon as the lines are uniformly black on the 
dial then have the patient look at the test letters again and 
increase the strength of the minus sphere if necessary until 
the eye can read ^J{ or ^J. Presuming that — 2 sphere and 

— 2 cylinder at axis 180 degrees were used, then the dif- 
ference between these and the -{• 4 sphere would give -f- 2 
cylinder, axis 90, as the amount of the hyperopic astigma- 
tism. 

How to Proceed in Compound Hyperopic Astigma- 
tism. — If the meridian of highest refraction is 4 D., then 
place a -f 5 D- sphere before the eye, wait a few seconds, 
then begin by adding minus spheres until a series of lines 
on the clock-dial show up very black, then add minus 
cylinders until all the lines become uniformly black, then 
turn to the test letters and increase the strength of the minus 
sphere as necessary until the vision is brought to normal. 
20 



234 REFRACTION AND HOW TO REFRACT. 

With -f 5 D. sphere before the eye and if — i sphere and 
— 3 cylinder at axis 165 were employed, then the com- 
pound hyperopia would be -f - l sphere with -f- 3 cylinder at 
axis 75. 

How to Proceed in Mixed Astigmatism. — Estimating 
the refraction approximately as +2O — 3, cylinder; place 
-f 5 D. sphere in position as before, then find the meridian 
of darkest lines on the clock-dial, add minus cylinder with 
the axis at right angles to the darkest lines, then when all 
lines are equally black, diminish the strength of the plus 
sphere until vision comes to the normal, if it is possible to 
get it to this point. The result may be -j- 5 sph. O — 
2.50 sph. O — 3-5° c yl- X 180, which would equal +2.50 
C — 3.50 cyl. X 180 . 

Advantages of the Fogging Method. — It is one of the 
best approximate ways of estimating the refraction if for 
any reason a cycloplegic can not be employed, as in cases 
of glaucoma, nursing mothers, or of an individual who may 
have an idiosyncrasy for a cycloplegic. The fogging method 
also has the advantage, like the cycloplegic method, of un- 
covering or bringing out most if not all the latent hyper- 
opia. 

Manifest or Dynamic Method. — This method is the 
very reverse of the "fogging" method, in that the refraction 
is estimated without first making the eye artificially myopic, 
or placing the ciliary muscle at rest. It is, therefore, a 
method that is liable to give all sorts of erroneous results 
if the subject is hyperopic and less than forty years of 
age. 

To estimate the refraction by the manifest method, the 
patient is told to look first at the test letters and then at the 
clock-dial at 6 meters distant; the astigmatism is corrected 
and then plus or minus spheres added as necessary to bring 



HOW TO REFRACT. 235 

the vision to the normal. The rule is to employ the 
strongest plus lenses or the weakest minus lenses which 
will give normal vision. 

Advantages of the Manifest Method. — This is the 
method by which the eyes of patients past forty-five years 
of age are refracted ; a fairly good method in cases of 
compound myopia in young subjects if drops can not be 
employed. 

Objections to the Manifest Method. — It is not a method 
to be used as a rule -in young subjects. If a young subject 
must be refracted without drops, then the fogging method 
should be followed. 

The habit of prescribing glasses from the manifest or 
"fogging" method without any knowledge of the ophthal- 
moscopic findings is not a method that merits the attention 
of the conscientious physician. Such work is very unsatis- 
factory, often leading to gross errors, ultimate dissatisfaction 
on the part of the patient, or injury to the eyes. 

Postcycloplegic Refraction. — The ordering of glasses 
after the static refraction has been recorded and the effect of 
the cycloplegic has left the eyes. For instance : While the 
ciliary muscle is paralyzed with atropin the static refraction 
is found to be +1.50 sph. O +2.00 cyl. axis 90 degrees, 
which gives a vision of — . The atropin is then stopped 
and the patient told to report in fifteen days, when the ciliary 
muscle will have regained its original strength and gone 
back to its old habit of accommodating for distance. The 
static refraction is placed before the eyes, and the strength of 
the sphere is gradually reduced until the vision just equals 
— , as it was when the " drops " were in the eyes. What- 
ever this correction with the glasses may be, is ordered. 
Occasionally the strength of the cylinder as well as its axis 
is also changed. 



236 REFRACTION AND HOW TO REFRACT. 

Objections to the Postcycloplegic Method. — The pa- 
tient is annoyed by the long delay to which he is subjected 
before getting his glasses. But the principal fault lies in 
the fact that the eye is not placed in the emmetropic condi- 
tion ; it is allowed to retain more or less of its accommo- 
dative power for distance. This, however, can not always 
be avoided. 

Static Refraction. — By this method the glasses are pre- 
scribed while the ciliary muscle is under the effect of the 
cycloplegic. In hyperopia allowance must be made in the 
strength of the sphere for the distance at which the test is 
made. At 6 meters 0.25 is deducted from the sphere 
without any change in the cylinder. The only possible 
objection to this method is in cases of hyperopia, in which, 
after the effect of the cycloplegic passes away, the ciliary 
muscle may endeavor to accommodate for distance with 
the glasses in position, and with the result that the pa- 
tient can not see clearly except near at hand. To avoid 
any such contingency the surgeon will have to make a 
deduction in the strength of the plus sphere to meet such 
cases. The rule for ordering glasses by the static refrac- 
tion in hyperopia is to deduct 0.25 from the sphere and 
have the glasses worn at once and constantly while the 
effect of the " drops " is gradually leaving the eyes. In 
this way the eyes grow accustomed (slowly) to seeing at a 
distance without exerting the ciliary muscle ; the eyes are 
thus placed in an emmetropic condition. If this effect of 
the cycloplegic passes away before the patient can obtain 
the glasses, it will be necessary to use the drops for a day 
or so after the glasses are received. Unfortunately, how- 
ever, some hyperopic eyes, in young subjects especially, 
with vigorous ciliary muscles, will develop a spasm of the 
accommodation for distant vision which will make the 



HOW TO REFRACT. 237 

glasses very annoying on account of distant objects look- 
ing "dim." Such patients should be advised of this fact 
at the time the glasses are ordered, and if dim distant 
vision does develop, that it will be transitory, and to per- 
sist in wearing the glasses. There are two ways of reliev- 
ing this " dim " vision if it should occur : 

1. To prescribe a weak solution of atropin (-^ of a grain 
to 1 fluidounce), I drop in each eye once or twice a day, 
the idea being to slightly relax the accommodation ; this is 
accomplished, but, unfortunately, the mydriatic effect is a 
disturbing element which the patient will not submit to 
long enough, as a rule, to obtain relief. 

2. The better way is to make a compromise in the 
strength of the sphere. An eye which has been in the habit 
of accommodating 3, 4, 5, or 6 diopters for distance, does 
not often give up this habit very gracefully, even if assisted 
by a slowly acting cycloplegic, so that when the static 
refraction calls for more than 3 diopters, the surgeon is fre- 
quently compelled to make a deduction of more than 0.25. 
There is no hard and fast rule as to just how much shall be 
deducted, and very few surgeons agree on this point. 
Glasses may be ordered as follows, the surgeon being 
guided in great part by the patient's age and occupation ; 
also as to whether there is esophoria or exophoria. It 
will be found that cases of esophoria will accept almost 
a full correction, whereas cases of exophoria will require a 
very liberal deduction in the strength of the glass, the 
patient being allowed to use his relative hyperopia : 

Static refraction at 6 meters : 

1. 00 sph. or less deduct 0.12 or 0.25. 

From -f- 1. 00 sph. up to 3.00 sph. " 0.25 or 0.50 or 0.75. 

" —3.00 sph. up to 6.00 sph. " 0.50 or 0.75 or 1. 00 or 1.50. 

" -j-6.00 sph. up to 8.00 and above " I. or 1.50 or 2.00 up to 3.00. 



238 REFRACTION AND HOW TO REFRACT, 

It is true that glasses ordered in this way do not leave 
the eyes in an emmetropic condition, and that, later on, when 
asthenopic symptoms redevelop, the strength of the glasses 
will have to be increased. But this method has two advan- 
tages : first, it gives the patient his glasses without any long 
delay, and the eyes have an opportunity to become accus- 
tomed to them while the effect of the " drops " is passing 
away ; and, second, the patient accepts a much stronger 
glass in this way than by the postcycloplegic method, which 
is a decided advantage. 

The ordering of lenses in low errors for distant vi- 
sion depends entirely upon the condition of the patient's 
eyes and symptoms. It is not unusual to find the most 
distressing asthenopia, headaches, blepharitis, etc., disap- 
pear as if by magic when corrections are ordered for small 
defects, especially if there is astigmatism. In other instances 
slight ametropic errors may not produce any unpleasant 
symptoms, and such a patient need not wear the correction 
for distance. 

The Ordering of Glasses in Myopia. — There is no 
fixed rule for prescribing glasses in myopia. Each case is a 
law unto itself, and should receive the most careful consid- 
eration from every point of view. But as the student must 
have some idea as to how to proceed, the writer would sug- 
gest the following subdivisions : 

T. Myopic eyes which can with safety use one pair of 
glasses for distant and near vision. 

2. Myopic eyes which require two pairs of glasses — one 
for distant vision, and another pair for near-work, reading, 
writing, etc. 

3. Myopic eyes which should have the near correction 
only. 

Class i comprises those cases in which there is an active 



HOW TO REFRACT. 239 

ciliary muscle, and the ophthalmoscope shows but little, if 
any, change in the eye-ground indicative of stretching (chil- 
dren, or in beginning myopia). Glasses carefully selected 
by the static refraction may be ordered in such instances 
for constant use, but with the distinct understanding that 
if any discomfort arises at any time they will be subject 
to change. 

Class 2. — Adults who have not previously worn their 
myopic glasses. In these cases the power of the ciliary 
muscle is weak, deficient in sphincter fibers, and, if forced 
into activity, the patient would be very uncomfortable, the 
eye would stretch, and the myopia increase, the tissues in 
these eyes yielding more readily than in class 1. The 
glasses selected by the static refraction may be prescribed for 
distance, but a second pair, 1, 2, or 3 diopters weaker, must 
be ordered for the reading, writing, or working distance, 
that the accommodative effort may in part be kept in 
abeyance. Class 1, if not carefully watched, may pass 
into class 2 and class 2 may pass into class 3. 

Class 3. — These cases require unusually strong lenses, 
and it is to these especially that the term " sick," or 
"stretched" eye particularly belongs. The ophthalmo- 
scope may show vitreous opacities, areas of retinochoroid- 
itis, macular choroiditis, a broad myopic conus, and even 
posterior staphyloma. The eyes are prominent, occupying 
much of the orbital space. Eyes of such length are limited 
in their power of comfortable rotation, and hence it is com- 
mon for one eye to diverge, the patient stating that he uses 
only one eye for near vision. The diverging eye is usually 
more or less amblyopic, due to want of use or pathologic 
changes, or to both. Such eyes have lost almost or quite 
all the power of accommodation. These eyes must be 
placed in such a condition that the desire to converge and 



24O REFRACTION AND HOW TO REFRACT. 

accommodate is at a minimum. The prescribing of glasses 
for these long eyes must be limited to the one pair for near- 
work, and yet the patient may, by bringing the glasses 
closer to the eyes, improve the distant vision for the time 
being — a sort of artificial accommodation. To appreciate 
what is meant by this statement it is necessary to reconsider 
the optics of a myopic eye. A myopic eye of 20 D. has a 
far point of 5 cm., and the minus lens required to make 
such an eye receive parallel rays of light at a focus upon 
its retina should be of such strength that the rays passing 
through it would have a divergence as if they came from 
this far point (5 cm.). Such a lens would be a — 20 D. 
This means, of course, that the — 20 D. would have to be 
placed with its surface against the surface of the cornea, 
which is an impossibility. The usual distance for a lens in 
front of the eye is 1 or 1 ^ cm., so that this distance must 
be subtracted from the distance of the far point. In this 
instance 1 cm. from 5 cm. would leave 4 cm., and this would 
represent 25 D. As just stated, the glasses for this class 
of patients are limited to the one pair for near-work, and 
therefore it would be necessary to reduce the strength of 
these lenses 4 diopters and thus prevent, as far as possi- 
ble, any accommodative effort. The patient using this 
— 21 D. for near, can, if he wishes, improve his distant 
vision at any time by pressing the lenses closer to his eyes. 
The strength of concave lenses increases as they are 
brought closer to the eyes, and diminishes as they are re- 
moved from the eyes. 

Caution. — The great danger in any refraction at the trial- 
case, but especially in myopia, is an overcorrection, and 
this is very likely to occur if the surgeon is not extraor- 
dinarily careful in having his lenses placed as close to the 
eyes as possible while making the test. 



HOW TO REFRACT. 24 1 

Prophylaxis. — The prescribing of glasses for myopic 
eyes is only a part of the general treatment to which these 
"sick" eyes are entitled. If the treatment stops at this 
point, then the glasses may be an injury instead of a bless- 
ing. Myopia once established may pass through the 
various classes already described, and eventuate in greatly 
reduced vision or total blindness if certain limitation of 
their use is not insisted upon. 

1. Light. — A good, clear, and steady light is always essen- 
tial ; it should come from the left side, never from in front. 

2. Time. — The length of time that myopic eyes may be 
used should be restricted as much as possible, consistent 
with their condition ; that is to say, they should never be 
used after they become the least fatigued, and any use of 
the eyes should be counteracted by life in the open air. 

3. Attitudes. — The head should have as little inclination 
as possible in reading, writing, or close work, as so faulty a 
position invites a congestion of the intraocular tissues. At 
school or at home the book should be inclined, and its dis- 
tance from the eyes be regulated by the size of the patient. 

4. Print. — The -use of small print or minute objects 
must be forbidden. English or Gothic type should be sub- 
stituted for Greek, German, and other characters. Fine 
needle-work, embroidery, etc., must be abolished. If nec- 
essary, music notes must be given up entirely. 

5. Health. — The health of the patient must be looked 
after, and all irregularities corrected — constipation, etc. 

These are a few of the major considerations to which the 
patient's attention must be drawn, the surgeon being limited 
in his remarks to the exigencies of the individual case. 

In conclusion it may be well to know how myopia is 
produced, since it has been stated that the condition is rarely 
seen in young children. It is well known that astigmatism 



242 REFRACTION AND HOW TO REFRACT. 

(hyperopic) is a congenital defect, and with this in mind it 
is very easy to appreciate the succeeding steps which lead 
to the compound myopic condition, showing at the same 
time the reason why simple myopia is so rare an anomaly. 

Take a child six years of age who has a compound hyper- 
opia of say -f-O.50 sph. O +0.75 cyl. axis 90 degrees ; this 
child enters upon its course of study without any correcting 
glasses, and is subjected in its pursuit for knowledge to a 
faulty school desk and chair, possibly facing a window. 
The print is defective in many ways. The artificial light 
for home study in the evening may be of poor quality, and 
so placed that but few of its rays fall upon the child's book. 
With these and other hindrances the eyes are strained 
(stretched). The tissues are very yielding in their growing 
state, so that at the age of ten years the refraction may 
show +0.75 cyl. axis 90. The +0.50 sph. (the axial 
ametropia) has disappeared by an elongation of the optic 
axis. The vertical. meridian is now emmetropic. The same 
conditions exist for the next three years, during which the 
number of studies is multiplied and the hours for study are 
prolonged and the child reaches the age of puberty ; the 
refraction is now found to be — 0.50 sph. O +0.75 cyl. 
axis 90 degrees, mixed astigmatism. In two years more the 
refraction is found to be — 0.25 sph. O — 0.75 cyl. axis 
180 — i. e. y compound myopic astigmatism. From this time 
forward these eyes progressively stretch and are subject to 
the stretching process unless the progress is stayed with 
glasses and prophylactic treatment. 

In the brief detail of this one case the student will fully 
appreciate another important fact — that the vertical meridian 
of the cornea, as a rule, maintains throughout the shortest 
radius of curvature. This is abundantly demonstrated by 
statistics. 



HOW TO REFRACT. 243 

The following summary of refractive errors and direction 
of meridians of shortest radius of curvature in 2500 pairs 
of eyes, — 1300 in private and 1200 in hospital work, — pre- 
pared by Dr. Risley and the writer, shows the correctness 
of the above statements : * 

Private. Hospital. 

Monocular astigmatism, 70 5.0% 94 7.8% 

Binocular astigmatism, 1151 88.5 828 69.0 

Total cases, 1 

Binocular symmetric astigmatism, 
Binocular asymmetric astigmatism, . 

Heteronymous astigmatism, 123 

Homonymous astigmatism, .... 

Total binocular astigmatism, . . 

Symmetric astigmatism : 

(a) According to rule (homologous), 
(6) Against rule (heterologous), . 

Total symmetric astigmatism, . . . 694 613 

Asymmetric astigmatism : 

(a) According to rule, 223 71.8% 126 79.1% 

(6) Against rule, 87 28.2 32 20.9 

Total asymmetric astigmatism, ... 310 158 

Direction of the Meridian of Shortest Radius in all Cases of 
Symmetric Astigmatism. 

Meridian at 90 , 57-o4-% 

Meridian inclined 15 or less on each side, I 9-7-h 

Meridian inclined from 15 ° to 30 on each side, 4.0 — 

Meridian inclined from 30 to 45 on each side, 1.0 

Meridian at 180 , 12.0 

Meridian inclined 15 or less on each side, 4.0 — 

Meridian inclined from 15 to 30 on each side, 2.0 

Meridian inclined from 30 to 45 ° on each side, 0.5 

* This report was read in the Section on Ophthalmology at the forty-sixth 
annual meeting of the American Medical Association, at Baltimore, Md., May 
7 to 10, 1895. 



1221 




922 




694 


60.2% 


613 


74-4% 


310 


26.8 


158 


19.8 


123 


10.6 


40 


5-2 


24 


2.1 


17 


1.2 


1151 




828 




543 


78.2% 


559 


97-7^ 


151 


21.8 


54 


2.3 



CHAPTER X. 
APPLIED REFRACTION. 

In estimating the refraction of any eye the surgeon will 
do good work if he will make it a rule never to be satisfied 
until each eye has a vision of — or more, and if this visual 
acuity is not attained, to understand the reason why : 
whether it is his fault or the fault of the eye itself. It is 
most essential in every instance to have the good-will of 
the patient. 

The following cases are detailed so as to demonstrate 
each form of ametropia in all its phases : 

Case I. — Simple Hyperopia. — This is a common form 
of ametropia, occurring about 20 times in 100 cases : 

January 3, 1899. John Smith. Age, twenty. Single. Stenographer. 
O. D.-jtf. p. p. = type 0.50 D. at 12^ cm. 

O. S. -Yy-. P- P- = type 0.50 D. at 12^ cm. 
Add. = 22 degrees; abd. = 6 degrees. 

History. — Frontotemporal headaches almost constantly, 
but much worse when using eyes at near-work. Had 
severe headaches when a school-boy. Never liked to 
study ; preferred out-of-door sports. 

S. P. — Face symmetric, but narrow. Blepharitis mar- 
ginalis. Irises blue. Pupils round, 3 mm. in diam. Eyes 
fix under cover. 

Ophthalmometer. — Negative. 

Ophthalmoscope. — Both eyes the same. Media clear. 

244 



APPLIED REFRACTION. 245 

Disc small and round, with physiologic cup. All vessels 
near the disc are seen clearly with -j- 2 S. Eye-ground 
flannel-red and accommodation very active. 
Manifest or dynamic refraction : 

O.D.+,.2 5 S.=^, 

O. S. +1.25 s. = ^-. 

R . Atropin and dark glasses for refraction. 

January 5, 1899. Patient seated at 6 meters from test- 
card and small point of light. O. D. V.===g==0. S. V. 



VI 
XXX' 



XXX 



Cobalt-blue glass shows (each eye separately) blue 
center and red halo. (See Fig. 125.) 

Retinoscope, with +3 S., developed point of reversal at 
1 meter for each eye. 

With trial-lenses, O. D. and O. S. each select +2 S., 
which gives a vision of —-, and they positively refuse to 
see -^- clearly with an addition of +0.25 S. In other 
words, this +2 S. is the strongest lens which each eye will 
accept and maintain clear distant vision. The rule for 
refraction in hyperopia is to employ the strongest lens 
which the eye will accept without blurring the distant 
vision. 

To prove that the ciliary muscle is at rest, and that the 
glass selected is correct, add a +4 S. to the distance cor- 
rection, and the rays of light emerging from the eye must 
focus at the principal focus of the added 4-4 S., at 10 
inches (25 cm.); and if the patient can read fine print at 
this distance, the ciliary muscle is at rest and the glass cor- 
rect. If a -f 3 S. had been added instead of a -f-4 S., then 
the principal focus would be at 13 inches; if +5 S., then 
at 8 inches, etc. 

The question is, What glasses shall be ordered ? The 



246 REFRACTION AND HOW TO REFRACT. 

writer would give the following prescription, and instruct the 
patient to stop the drops and wear the glasses constantly : 

For Mr. Smith. 

Be. O. D. +1.75 sph. 
O. S. +1-75 s ph- 

Sig. — For constant use. 
January 7, 1899. 

January 8th : Glasses from the optician neutralize ; are 
centered and accurately adjusted. 

January 21st : Add. =18 degrees. Abd. = 6 degrees. 
Near point in each eye = 10 cm. No headache or dis- 
comfort of any kind. 

Considerations. — The static refraction as represented by 
-f-2.00 sph. means that rays of light which pass through 
this lens and focus at the fovea diverge from six meters' 
distance, which heretofore we have considered for purposes 
of calculation as parallel ; but when glasses are ordered, 
allowance must always be made for this small amount of 
divergence, and so 0.25 is deducted from the -j-2 sph., that 
the eye may have parallel rays focusing on its retina when 
looking beyond a distance of six meters. To have been 
mathematically exact, -J-o. 12 should have been deducted in 
place of +0.25. 

The purpose in all cases of refraction is to place the eye 
in an emmetropic condition, though this is not always advis- 
able in every instance. The hyperopic eye naturally accom- 
modates for distance, and the emmetropic eye does not ; then 
the hyperopic eye is made emmetropic when a spheric lens 
permits parallel rays to focus upon its fovea without any 
assistance whatever from the ciliary muscle. 

Advantages of Atropin in This and Similar Cases. — 
The glasses are ordered while the ciliary muscle is at rest. 
The accommodation returns gradually. The eye becomes 



APPLIED REFRACTION. 2Afi 

accustomed to seeing at a distance without the assistance 
of the ciliary muscle. Atropin produces a physiologic rest, 
which the overacting ciliary muscle, disturbed choroid, and 
irritated retina require. None of these good results can be 
expected in a case of this kind from the use of a " quick " 
cycloplegic like homatropin. 

Summary. — Age of patient, twenty years. Amplitude of 
accommodation is 10 D. for this age. 

Near point is 12 J cm., which shows only 8 D. 

Facultative hyperopia (Hf.) equals difference between 10 
and 8D., which is 2 D. 

Manifest hyperopia (Hm.) equals 1.25 D. 

Total hyperopia, or static refraction (Ht.), equals 2 D. 

Latent hyperopia (HI.) equals the difference between the 
manifest and total, 2 D. and 1.25 D., making 0.75 D. 

The far point, or conjugate focus, is negative or virtual, 
and lies back of the retina, where the emergent rays (diverg- 
ing) from the eye would meet if projected backward; this 
point corresponds to the principal focus of the lens which 
corrects the hyperopia — i. e., in this instance, +2 D., and 
the negative far point is therefore at 20 inches. 

The +1.75 makes the eye practically emmetropic; the 
near point, after the effect of the atropin passes away, is 10 
cm., which is the emmetropic near point for the patient's 
age. The plus sphere selected represents a shortening of 
the eye of § mm., as measured on the optic axis. 

Case II. — Simple Myopia. — With the one exception of 
emmetropia, it will be found that myopia, plain and simple, 
without astigmatism, is one of the rarest conditions of the 
eye which the surgeon will meet in careful refractive work. 
About one and one-half per cent, of all patients, by careful 
refraction, are found to have simple myopia. Therefore 
the condition is not common. 



248 REFRACTION AND HOW TO REFRACT. 

January 3, 1899. Miss Rare. Age, twenty-five years. Single. 

O. D. V. = -y~- P- P- = tyP e °- 2 5 -D. at 9 cm.; p. r. at 40 cm. 

O. S. V. = jjT. p. p. = type 0.25 D. at 9 cm.; p. r. at 40 cm. 
Add. 16 degrees. Abd. 6 degrees. Exophoria, 3 degrees at 13 inches. 

History. — Does not suffer much from headache, but eyes 
ache after any prolonged use at near-work. Never able to 
see well at a distance. Always stood high in her class at 
school, though she had to have a front seat to see the fig- 
ures and writing on the blackboard. Has excellent vision 
for near-work and does fine embroidery. Has been accused 
of passing friends on the street without speaking to them. 
If she drops a pin on the floor, has to get on her knees to 
find it. Parents do not wear glasses. Grandfather had 
"elegant" sight, had "second sight," and never wore 
glasses. Patient has postponed getting glasses because 
parents objected. 

6*. P. — Face symmetric. Interpupillary distance, 65 mm. 
Irises dark. Pupils large, round, 5 mm. Eyes out under 
cover. 

Ophthalmometer. — Negative. 

Ophthalmoscope shows each eye the same. Media clear. 
Disc large and round. Shallow physiologic cup. Narrow 
myopic conus at temporal side of disc. Choroidal vessels 
seen throughout periphery of eye-ground and extending 
almost to nerve margin. All vessels near nerve-head seen 
with — 3. S. 

Manifest Refraction. — Each eye — 3.00 sph. gives vision 
~r VI 

Cobalt-blue glass gives red center and dark blue halo. 
(See Fig. 126.) 

R. Atropin and dark glasses for refraction. 



APPLIED REFRACTION. 249 

January 5, 1899: Patient seated at 6 meters from test- 
card. O. D. and O. S. vision equals -^j-. Retinoscope with 
— 1.50 S. develops point of reversal at 1 meter for each eye. 
It will be noticed that the vision" in hyperopia with and 
without drops is decidedly different, whereas in myopia 
there is little, if any, change. With trial-lenses each eye 
selects separately — 2.50 sph., which gives a vision of 



VI 
VI ' 



VI 



If a — 2.25 sph. is substituted, the vision falls to 7777^. If a 



— 2.75 is employed, the vision remains -^-, but the letters 
look small, black, and "far away." The rule for refraction 
in myopia is to employ the weakest lens through which the 
eye can still maintain clear, distant vision. 

What Glass to Order. — The writer would give the fol- 
lowing prescription and instruct the patient .to stop the 
drops and wear the glasses constantly : 

January 7, 1899. For Miss Rare. 
R. O.'D. —2.50 sph. 
O. S. — 2.50 sph. 
Sic. — For constant use. 

January 9th : Glasses neutralize ; are centered and accu- 
rately adjusted. Add. 18 degrees. Abd. 10 degrees. 
Patient is delighted with glasses. 

January 21st: Near point, 12 cm. 

Considerations. — As a rule, concave lenses are ordered 
without any deductions for the slight amount of diver- 
gence of the rays of light for the distance (6 meters) at 
which the estimate is made. To be exact, — 0.25 should 
be added to the — 2.50 sph. in this case ; but the surgeon 
must avoid the danger of ^wrcorrecting the myopic eye, 
and, to be on the safe side, the glass is usually ordered as the 
patient selects and the retinoscope confirms it. These lenses 
make the eyes, to all intents and purposes, emmetropic. 



25O REFRACTION AND HOW TO REFRACT. 

Advantages of Atropin. — The choroid and retina are 
given a physiologic rest that they could not obtain in any 
other way. The patient will not select too strong a glass, 
as was the case in this very instance when manifested. 
Myopic eyes usually have large pupils, and do not suffer, 
therefore, from mydriasis to the same extent as hyperopic 
eyes. The far point remains unchanged. The power of con- 
vergence is somewhat relieved by the glasses, which at 
the near working distance are of the nature of prisms, 
bases in. 

Summary. — Age of patient is twenty-five years. Ampli- 
tude of accommodation at this age is 8.5 D. Near point, 9 
cm. = 11 D., and far point 40 cm. = 2.50 D. Difference 
between near point and far point in diopters = 8.50 D., which 
is the amplitude of accommodation for the patient's age. 

Difference between the near point in diopters (11 D.) 
and the amplitude (8.50 D.) is 2.50 D., which is the amount of 
the distant correction needed. With glasses on, the near 
point, after the effect of the atropin passes away, is 12 cm., 
which represents the emmetropic near point for the age. 
This myopia of 2.50 D. represents an eye nearly 1 mm. longer 
than the standard eye, as measured on the optic axis. 

Case III. — Simple Hyperopic Astigmatism. — Not an 
uncommon condition. About 5.5 per cent, of all eyes have 
this form of refraction. 

April 3, 1899. Miss Robinson. Age, twenty-four years. Single. Dress- 
maker. 
O. D. V. = -§r ? ? ?. p. p. = type 0.50 at 13$ cm. 

O. S. V. = ~ ? ? ?. p. p. = type 0.50 at 13 J cm. 
Add., 23 degrees. Abd., 5 degrees. At 6 meters, esophoria 4 degrees; 
at 13 inches 10 degrees of exophoria. 

Astigmatic clock-dial shows darkest lines from X to 
IV with O. D. 



APPLIED REFRACTION. 25 I 

Astigmatic clock-dial shows darkest lines from VIII to 
II with O. S. 

History. — Headache every day ; seldom entirely free from 
ocular discomfort. Distress begins in the forehead and 
extends to the back of the head and into the neck. After 
a hard day's sewing, has to go to bed and bind the head 
with a handkerchief. Once a week has a " sick headache," 
when she has to give up work entirely and take headache 
powders. Sick headache often ceases after emesis. 

5. P. — Face symmetric. Blepharitis well marked, with 
many cilia missing. Edges of lids thickened. Irises light 
blue in color. Pupils apparently oval in vertical meridian. 
Corneal reflex shows axis inclined from vertical in each eye. 

Ophthalmometer. — O. D., 1.50; axis, 75, with the rule; 
O. S., 1.50; axis 105 degrees, with the rule. 

Ophthalmoscope. — O. D., vertically oval nerve axis, 75 
degrees. Accommodation very active. Underlying conus 
down and out. Vessels at 75 best seen with +1.50; and at 
axis 165, without any lens. O. S., same general conditions 
as in O. D. Vertically oval nerve axis, 105. Vessels at 105 
degrees seen with +1.50, and at axis 15 without any lens. 

M an i jest Refraction. — 

O. D., + 1.25 cyl. axis 65 degrees = -r=- ???. 
O. S., same cylinder with axis 125 degrees. 
rj . Hyoscyamin and dark glasses for refraction. 

April 5th : Six meters from test-card and point of light. 
O. D. V. = ^???. 

o. s. v.=^???. 

Clock-dial shows the same as at first examination. 
Cobalt-blue glass before O. D. gives blue center and red 
on each side at axis 165 degrees. O. S. the same at axis 
15 degrees. (See Fig. 128.) 



252 REFRACTION AND HOW TO REFRACT. 

Stenopeic Slit. — O. D., axis 75 with +0.25 S., V. = ~\ 
and at axis 165 with +1.50 S., V. = —-. O. S., axis 105 



VI 



VI 



with +0.25 S., V. = — ; and at axis 15 with +1.50 S., 

V = -^ 

vi ' 

Retinoscope at 1 meter shows : O. D., at axis 75 degrees 
+ 1.25 S., and axis 165 degrees, -f 2.25 S. O. S., at axis 
105 degrees, +1.25 S., and at axis 15 degrees, +2.25 S. 

At Trial-case. — 

O. D. -j-0.25 sph. O -f-i.oo cyl. axis 75 degrees, V. = _XI -f- 
O. S. -j-°- 2 5 sph. O -j-i-OO cyl. axis 105 degrees, V. = VI -\-. 

April 6th : Same result as April 5th. Add., 20 degrees. 
Abd., 6 degrees. Esophoria, 2 degrees at 6 meters. 

For Miss Robinson. 

R. O. D. -f-i-°° cyl. axis 75 degrees. 
O. S. -f 1.00 cyl. axis 105 degrees. 
Sig. — For constant use. 

April 7th : Glasses neutralize ; are centered and accu- 
rately adjusted. 

April 1 6th : Perfectly comfortable. Free from headache 
since the first day she used the " drops." Add., 20 de- 
grees. Abd., 6 degrees. Esophoria at 6 meters, 2 de- 
grees ; and at 13 inches, o°. Near point, 12 cm. 

Considerations. — Apparently, the static refraction in this 
case would indicate compound hyperopic astigmatism ; but 
when 0.25 is deducted to produce parallel rays, then the 
prescription becomes one for simple hyperopic astigmatism. 

General Rule for Prescribing Cylinders. — Order the 
cylinder just as found, without any change in its axis or 
strength. 

The vision in each eye at the different visits, before 
lenses were placed in front of the eyes, was always uncertain, 



APPLIED REFRACTION. 253 

the patient miscalling certain letters, and hence it is that 
the vision is recorded with as many question marks as 
there are mistakes in the line of letters — i. e.. -rnr ? ? ?. 

IX 

In taking the vision at the first visit, the patient could 
read part of ^g^ if not closely watched. In other words, 
if she was permitted to tilt her head to one side and nar- 
row the palpebral fissure by squinting the lids together, and 
making, as it were, a stenopeic slit out of her eyelids, the 
vision was improved. But when told to open the eye wide, 
she could read only part of —-. This is explained by 
the fact that when the lids were drawn together, the verti- 
cal meridian was partly excluded, and then, by accommo- 
dating, the vision was improved through the horizontal 
meridian. Astigmatic eyes often take advantage of this 
condition when the nature of the astigmatism is suitable, 
but only at the expense of frowning and straining the 
accommodation. 

It will also be noticed that the stenopeic slit was not used 
as a test at the first visit. This is also explained for the 
same reason that the patient would draw his lids together 
and therefore annul the virtue of this test. The stenopeic 
slit is to be used in these cases only when the ciliary muscle 
is at rest. 

Summary. — When the hyoscyamin has passed out of 
the eyes and the glasses are in position, the near point be- 
comes 12 cm., which is quite consistent with the patient's 
age. Before using drops, the near point with the eyes wide 
open was only 13! cm., representing about 2.50 D.; and 
this, subtracted from the amplitude for twenty-four years of 
age, would leave 1 D. for distance uncorrected. 

As every 6 D. cylinder represents 1 mm. of lengthening 
or shortening of the radius of curvature of the cornea, 
then this patient, taking a +1 cyl. at axis 75 in the right 



2 54 REFRACTION AND HOW TO REFRACT. 

eye, has the 165 degree meridian J of a mm. too long as 
compared with the 75 meridian, which is supposed to have 
the normal radius of 7.8 mm. 

The same is true of the meridians of the left eye. 

Case IV. — Simple Myopic Astigmatism. — Not a com- 
mon condition. About 1.5 per cent, of all eyes have this 
form of refraction. 

April 10. Miss Jenks. Age, eighteen years. Single. 
O. D. V. ="V~ ???• P- P- 9 cm. P- r - 50. cm. 
O. S. V. = -^-???. p.p. 9 cm. p. r. 50. cm. 

Add. , 20 degrees. Abd. , 5 degrees. Esophoria at 6 meters = 3 degrees ; 
and 1 degree at 13 inches. 

Pointed Line Test. — Each eye selects the series of points 
from XII to VI as coalescing and appearing as dark 
lines. 

Cobalt-blue Glass. — O. D. and O. S. each show blue 
above and below the red. (See Figs. 129 and 130.) 

Stenopeic Slit. — Axis 90 degrees V. = ~^; axis 180 V. 

- -¥L 

vi *■ 

History. — Never had good distant vision. Has occa- 
sional headaches. Comes to find out if glasses will im- 
prove vision. 

S. P. — Face symmetric. Irises dark in color. Pupils 
apparently round, 5 mm. in diameter. Eyes out under 
cover. 

Ophthalmometer. — Each eye 2 D., axis 90. 

Ophthalmoscope. — O. D., media clear. Disc large and 
round, with underlying conus out. No physiologic cupping. 
Choroidal circulation everywhere recognized, characteristic 
of a stretching eyeball. Horizontal vessels seen with — 2; 
vertical vessels seen without any correcting lens. O. S., 
same general conditions as in O. D. 



APPLIED REFRACTION. 255 

Manifest Refraction. — 

VI 
O. D., —2.50 cyl. axis 180 = -yj-. 

O. S., —2.50 cyl. axis 180 = —-. 

R . Atropin and dark glasses for refraction. 

April 1 2th : Six meters from test-card and point of light. 
Retinoscopy at one meter. Vertical meridian — i.oo S. 
Horizontal meridian +1.25 S. 

Stenopeic slit at axis 1 80 with -j-o. 25 = — ; at axis 90 = 

— :>u, VI . 

Cobalt-blue glass and pointed line test show same results 
as at first visit : 

O.D.V.=-xv-???. 
O. S. V. =^???. 

At Trial-case. — 

VI 

O. D., -^0.25 sph. 3 — 2.25 cyl. axis 1 80 degrees == -yj-. 

VI 

O. S., +0.25 sph. 3 — 2 - 2 5 c )'l- ax i s 1 80 degrees z=-y|-. 

April 13th: Same results as yesterday. Add. = 20 ; 
Abd. = 6. Esophoria, 2 degrees. 

For Miss Jenks. 

ri. O. D., — 2.25 cyl. axis 180 
O. S., — 2.25 cyl. axis 180. 

April 14th : Glasses neutralize. Centered and properly 
adjusted. 

April 28th : Comfortable. Enjoys good distant vision. 
Near point, each eye, 9 cm. 

Considerations. — Apparently, the static refraction would 
indicate mixed astigmatism, but when -fo.25 is deducted 
to produce parallel rays, the prescription resolves itself into 
one for simple myopic astigmatism. 

The general rule for ordering cylinders is the same in 



256 REFRACTION AND HOW TO REFRACT. 

myopia as in hyperopia — i. e., no change in the strength or 
in the axis of the cylinder. - 

A cycloplegic is always necessary in such cases, as is 
shown by the different lenses obtained by the manifest and 
stenopeic slit. 

The vision was always uncertain before lenses were placed 
before the eyes, as is indicated by the question marks. 

At the first visit the vision was taken with the eyes wide 
open. If allowed to narrow the palpebral fissure by squint- 
ing the eyelids together and making a stenopeic slit out of 
them, the patient could read a part of ^7^. When the 
lids were thus drawn together, the myopic vertical meridian 
was excluded in part and the horizontal meridian was util- 
ized. The stenopeic slit was of some assistance before 
drops were used, as the accommodation could not be 
exerted, as in the case of the hyperope. 

Summary. — After recovery from the cycloplegic, small 
type was clear at 9 cm., which was in keeping with the 
patient's age. The near point before "drops" were used 
was also 9 cm., but not constant, nor was the type clear. 
The — 2.00 cyl. at axis 180 represents ^ of a mm. of 
shortening in the vertical radius of curvature as compared 
with the normal radius of 7.8 mm. in the horizontal. 

The astigmatism is regular, symmetric, with the rule. 

Case V. — Compound Hyperopic Astigmatism. — The 
most common form of all refraction. It is a combination 
of simple hyperopia with simple hyperopic astigmatism. 
About 44 per cent, of all eyes have this form of refraction. 

Bookkeeper. 



2 degrees ; at 13 



April 1 2th. 


Mr. Common. 


Age ; 


, twenty- eight. 


Married, 


O. D. 


V. 


=*»• 


P- 


p. = 


type 0.75 


D. 


=== 18 cm. 


0. S. 


V. 


=£"■ 


P- 


p. = 


type 0.75 


D. 


= t8 cm. 


Add., 


23 


degrees. 


Abd., 7 


degrees. 


Ei 


iophoria, 2 


inches, 


0. 













APPLIED REFRACTION. 25/ 

Astigmatic Clock-dial. — O. D. and O. S. each selects 
darkest series of lines from IX to III. 

Placido's disc shows each corneal image as a horizontal 
oval. Schemer s test shows two lights, separated in all 
meridians : the vertical have the least separation and the 
horizontal the most. 

Ophthalmometer. =2. 25 D. with axis 90 in each eye. 

History. — Family physician has tried in vain to stop the 
headaches, which he said were from biliousness. Headache 
develops as soon as the patient commences to use his eyes, 
and gets worse toward noon ; and from that time on, during 
the rest of the day, he is cross and irritable, and feels dizzy. 
Unable to read in the evenings as he did a few years 
ago. Is wearing a pair of " rest " glasses, which he received 
from an optician ; they were of some benefit for a very short 
time. 

5. P. — Lid margins red and excoriated. Many fine scales 
(looking like dandruff) adhering to the cilia. Irises gray 
in color. Pupils round, 3 mm. Eyes in, under cover. 

Ophthalmoscope. — O. D. and O. S. each medium clear. 
Disc small, vertically oval. Shallow physiologic cup. 
Venous pulsation on disc. Narrow conus to temporal side. 
Xerve-head prominent and edges somewhat hazy. No path- 
ologic conditions recognized. Vertical vessels best seen 
with +2.00 and horizontal with +0.50. 

1^. Duboisin and dark glasses for refraction. 

April 14th: Six meters from test-card and point of light. 
O.D.V.— g???. 
O.S.V.— S???. 

Retinoscopy develops point of reversal at one meter in 
each eve; vertical meridian with +1.75 S., and horizontal 
meridian with +3.50 S. 



258 REFRACTION AND HOW TO REFRACT. 

Stenopeic slit axis 90 degrees with + 1.00 = -^; at axis 
180 degrees with +2.50 — 



vi * 



Cobalt-blue glass, blue center and red all round, more 
conspicuous on the right and left sides. (See Figs. 131 and 
132.) 

At Trial-case. — 

O. D., +0.75 sph. C + 1.75 cyl. axis 90 = —-. 
O. S., +0.75 sph. C + i-75 cyl. axis 90 = -^p 

April 15th: Same results as April 14th. Add., 22. Abd. 
7. Esophoria, 1 degree. 

For Mr. Common: 

ty. O. D., +0.50 sph. O +i-75 cyl. axis 90 degrees. 

O. S., +0.50 sph. O +1.75 cyl. axis 90 " 
Sig. — For constant use. 

April 1 7th : Glasses neutralize ; are centered and ad- 
justed. 

April 24th : Has been perfectly free from headaches 
ever since getting glasses. Never realized what a blessing 
glasses could be. Near point with each eye is now 14 cm. 

Considerations. — The prescription for glasses was the 
same as the static refraction, with the exception of the re- 
duction in the strength of the sphere. No change in the 
cylinder. A cycloplegic as a means of obtaining a prompt, 
correct, and satisfactory result in such cases can not be dis- 
pensed with. 

The decided change in the vision before and with the 
cycloplegic is quite diagnostic of compound hyperopic as- 
tigmatism. When the cylinder and sphere are of any con- 
siderable strength, the patient can often overcome (faculta- 
tive hyperopia) the spheric but not the cylindric correction. 

Summary. — After recovery from the cycloplegic the 



APPLIED REFRACTION. 259 

small type becomes clear at about 13 cm., which is the 
near point consistent with the patient's age. The far point 
before using drops was really two points, both negative — 
that of the vertical meridian being about 2 meters, and the 
horizontal meridian about J of a meter back of the retina. 

The form of the astigmatism is regular, symmetric, and 
with the rule. 

Case VI. — Compound Myopic Astigmatism. — A com- 
bination of simple myopia with simple myopic astigmatism. 
This is the usual form of refraction in myopic eyes. About 8 
per cent, of all eyes have compound myopia. The writer's 
experience is such that he never refracts a case of myopia 
without searching carefully for a cylinder in combination 
with the sphere. (See page 123.) 

April 1 2th. Mrs. Usual. Age, thirty years. Married. Housewife. 
O. D. V., -j- ???, type 0.50 = 10 to 33 cm. 

O. S. V., — j— ???, type 0.50 = 10 to 33 cm. 
Add., 16 degrees. Abd., 6 degrees. Exophoria at 6 meters, 2 degrees. 

History. — Suffers from ocular pains, as if knife -points 
were sticking into the eyes, which come on as soon as near- 
work is attempted or continued. Says that she constantly 
sees fine dust particles floating before her vision. Has been 
wearing glasses from an optician ( — 3 sph.). Has all the 
symptoms of near-sightedness. Family history of father 
and two sisters wearing glasses for "near sight." 

S. P. — Face symmetric. Eyeballs prominent. Irises 
dark in color. Pupils small (for a myope) and round, 
3 mm. Eyes markedly out under cover. 

Ophthalmoscope. — O. D., many fine floating vitreous 
opacities. Nerve large and round, with broad underlying 
conus down and out. Choroidal vessels seen throughout 
eye-ground. Vessels at about axis 1 20 degrees best seen 



26o REFRACTION AND HOW TO REFRACT. 

with — 2, and vessels at axis 30 degrees best seen with — 3. 
O. S., same general conditions as in O. D., except the prin- 
cipal meridians are about 60 degrees and 150 degrees. 

Indirect method shows a vertically oval nerve, with the 
conus to the nasal side of the aerial image (as the eye- 
ground and nerve-head have undergone vertical and lateral 
inversion). Withdrawing the lens, the nerve grows larger 
in all meridians, but more so in the vertical. 

Cobalt-blue glass shows O. D. red center, blue all around, 
more pronounced on the sides in the 120 meridian. O. S. 
shows red center, blue all around, more pronounced on the 
sides in meridian of 60 degrees. (See Figs. 133 and 134.) 

Stenopeic slit before O. D. at axis 120 V. = ^; at 
axis 30 V. = -^??« O. S., the same with axis 60 degrees 
and 150 degrees. 

Astigmatic Chart. — O. D. selects the lines from V to XI 
as darkest. O. S. selects the lines from VII to I as 
darkest. 

Ophthalmometer. — O. D., 1 D., axis 35 degrees. O. S., 
1 D., axis 145 degrees. 

Manifest. — 

VI 
O. D., — 2.5osph. O—0.75, cyl. axis 35 = -yif ? ? ? ?. 

VI 

O. S., — 2.50 sph. O — 0.75 cyl. axis 145 = yjj- ? ? ? ?. 

R . Atropin and dark glasses for rest and refraction. 

April 13th: At six meters from test-card and point of 
light : 

O.D.V.=^+. O.S.V.--S + . 

O. D., — 2.00, sph. O — 1. 00 cyl. axis 30 = -™-. 
O. S., — 2.00 sph. C — i- 00 cyl. axis 150 = -==-. 

Retinoscope confirms this trial-case result. Retinoscope 
also shows a general cloudiness of. the media (vitreous), 



APPLIED REFRACTION. 26 1 

which, of course, will account in part for the vision not 
being -^- in each eye with correcting glasses. 
April 14th : Same result as on the 13th. 

For Mrs. Usual. 

R. O. D., — 2.00 sph. O — 1. 00 cyl. axis 30 degrees. 

O. S., — 2.00 sph. O — 1. 00 cyl. axis 150 " 
Sig. — For distance, as directed. 

April 15th: 

R . Tonics. Rest of eyes. Attention to general health. 

April 1 7th : Glasses neutralize ; are centered and ad- 
justed. 

April 29th : Add., 16. Abd., 6. Exophoria, 2. 

Vision in each eye -?r-, read slowly. 

Considerations. — The static refraction was ordered just 
as found, and no deduction whatever was made in the 
sphere. The rule is to prescribe in the same way as in 
simple myopia. But all cases of myopia can not and must 
not be prescribed for by rule. Each case of myopia is a 
law unto itself. See description under General Considera- 
tions, page 238, also pages 227, 228, and 229. 

Summary. — The near point is now 14 cm., which is 
perfectly consistent with the patient's age. Fourteen centi- 
meters represents an accommodative power of 7 D., and 
this was the difference between the near and far points 
before the drops were used. The astigmatism is regular, 
symmetric, and with the rule. Vision is not brought up to 
normal on account of the changes in the vitreous and dis- 
turbed eye-ground, due, no doubt, to the want of a proper 
correction — the cylinder. The choroid and retina are both 
in a stretching condition. 

Case VII. — Mixed Astigmatism. — Not an uncommon 
condition. About 6 l / 2 per cent, of all eyes have this form 



262 REFRACTION AND HOW TO REFRACT. 

of refraction. This is a combination of the simple hyper- 
opic and simple myopic astigmatisms, with their axes oppo- 
site or at right angles to each other, as a rule. 

April 8th. Mr. Crook. Age, twenty-one years. Single. Clerk. 

VI 
O. D. V. = -j^. p. p. = 12 cm. with type 0.75 D. 

VI 
O. S. V. = -^y\ p. p. = 12 cm. with type 0.75 D. 

Add., 20. Abd., 10. 

History of poor sight all his life, but thinks it was better 
as a boy. Has frequent frontotemporal headaches, which 
are worse after using eyes at any prolonged near-work. 
Father has good sight, but his mother and her family have 
all been near-sighted ; has one aunt that developed cata- 
racts. Has been to several "stores," but could not get 
fitted with glasses that would improve his vision. 

5. P. — Face broad and symmetric. Long interpupillary 
distance. Irises dark in color. Pupils large, 5 mm. ; 
round. Eyes out under cover. 

Ophthalmoscope. — O. D., media clear. Disc vertically 
oval, axis 105. Macular region shows changes. Vessels 
at 105 best seen with -j-2, and at right angles with — 2. 
O. S., same conditions found, except that the meridians are 
at 75 degrees and 165 degrees. 

Ophthalmometer. — O. D., 4 D. axis 100. O. S., 4 D. 
axis 75. 

Cobalt-blue Glass. — O. D. violet center, blue above and 
below in meridian of 105 and red at the sides in meridian 
of 15. O. S., pink center, blue above and below in meridian 
of 75, and red on sides at axis 165. (See Figs. 135 and 

1360 

Stenopeic Slit. — O. D. axis 15 with +2 sph., V. = -~] 
at axis 105 with — 2 sph., V. = ~. O. S. axis 165 with 
+ 2 sph., V. = -§-; at axis 75 with — 2 sph., V. = -^k 



APPLIED REFRACTION. 263 

Indirect Method. — Each eye shows a lengthening of 
the vertical meridian as the condensing lens is withdrawn 
from the eye, and at the same time the horizontal meridian 
grows narrower. As the lens is advanced toward the eye 
the vertical meridian grows shorter and the horizontal 
meridian grows broader. 

The astigmatic chart does not show any difference in the 
shading of the lines ; they all appear about the same, 
Retinoscope at I meter distance shows myopia in the verti- 
cal meridian and hyperopia in the horizontal. 

R . Atropin and dark glasses for refraction. 

April 10th : At six meters from test-card and point of 
light. O. D. and O. S. V. = ^. 

Cobalt-blue glass shows the same as at first visit. 

Retinoscope at the distance of one meter shows point of 
reversal in O. D. at axis 105 degrees with — 1.50D., and 
axis 15 with +3 D. O. S., axis 75 with — 1.50 D., and 
axis 165 with +3 D. 

Stenopeic Slit. — O. D., axis 15 with -f-2 sph., V. — 
~~; axis 105 with — 2.50. sph., V. = -—. O. S., axis 
165 with -\-2 sph., V. = -^jr; at axis 75 with — 2.50 sph., 
V = ^-. 

IX 

At Trial-case. — O. D, — 2.50 cyl. axis 15 degrees O 
+2 cyl. axis 105 degrees, V. = ^~. O. S., — 2.50 cyl. 
axis 165 O + 2 cyl. axis 75, V. = ^. 

Or, 

O. D., — 2.50 sph. O 44.50 cyl. axis 105, V. — vnss . 
O. S., —2.50 sph. O -f4.So cyl. axis ' 75, V. = ^iss- 

Or, 

O. D. -1-2 sph. O — 4.50 cyl. axis 15 degrees, V. = VIISS -f-. 
O S. —2 sph. O — 4.50 cyl. axis 165, V. — y IISS -\~. 



264 REFRACTION AND HOW TO REFRACT. 

April nth: Same results as April 10th. Add., 20. 
Abd., 8. 

For Mr. Crook. 

ft. O. D. -\-2 sph. O — 4.50 cyl. axis 15 degrees. 

O. S. -j-2 sph. O — 4.50 cyl. axis 165 " 
SlG. — For constant use. 

April 1 2th : Glasses neutralize ; are centered and adjusted. 

April 26th : Near point 10 cm., which is consistent with 
age of patient. 

Considerations. — The ophthalmoscope, retinoscope, 
cobalt-blue glass, indirect method, and stenopeic slit were 
direct guides to the character of the refractive error. 
Emphasis is placed upon these different methods, as so many 
beginners in ophthalmology have a fear or dread of the re- 
sult in refracting cases of mixed astigmatism. 

The stenopeic slit shows a difference of 4.50 in the two 
principal meridians ; bearing this fact in mind, if a — 4.50 
cylinder at axis 1 5 be placed before the right eye, then all 
meridians would be made equally hyperopic 2 D. Com- 
bining -f- 2 sph. with the — 4. 50 cylinder at axis 1 5 in the 
right eye or at axis 165 in the left, the refraction would be 
corrected. 

Or, if a +4-50 cylinder at axis 105 be placed before the 
right eye, then all meridians would be made myopic 
2.50 D. Combining a — 2.50 sph. with this +4.50 cylin- 
der at axis 105 in the right eye or at axis 75 in the left eye, 
the refraction would be corrected. 

For a further consideration of combination of lenses see 
page 51. 

The rule for ordering cylinders is the same in mixed 
astigmatism as in other cylindric corrections — without 
change. 

Summary. — The character of the astigmatism is regu- 



APPLIED REFRACTION. 265 

lar, symmetric, and with the rule. The near point returns 
to the normal for the age. Eyes with such high errors do not, 
as a rule, obtain a visual acuity of -^-, for the reason that 
changes have taken place in the eye-ground, especially at 
the macula. 

Case VIII. — Irregular Astigmatism. — 

April 2d. Mary Smiles. Age, ten years. Scholar. 

VI 
O. D. V. = xxx slowly. No p. p. obtained. 

O. S. V. = jj^-. Xo p. p. obtained. 

History of poor sight ever since an attack of measles 
when two years of age, at which time was kept in a dark 
room for six weeks. Eyes were never strong afterward ; 
always very sensitive to light. Child was sent home from 
school with a note from the teacher : " Mary is near- 
sighted and should see a doctor." 

S. P. — Eyelids appear normal. Excessive epiphora. 
Corneas nebulated, especially O. S., which has a decided 
leu coma at the pole. Anterior chambers of normal depth. 
Pupils 3 mm., round. Corneal reflex very irregular. 

Ophthalmoscope. — No view obtained of the eye -ground 
through the small pupils on account of corneal opacities. 
Homatropin mydriasis shows O. D. cornea faintly nebu- 
lated in scattered areas ; rest of media clear. Nerve small 
and round. Vessels at axis 35 degrees best seen with -\-2 
D. O. S., there is a 3 mm. area of opacity at the pole of the 
cornea ; no clear view of the eye-ground. Indirect method 
shows a small nerve and refraction hyperopia 

R . Atropin and dark glasses for refraction. 

April 22d : Retinoscope at 1 meter shows band of light 
at axis 35, indicating hyperopia. Other meridians very 
irregular. O. S., nothing definite made out. 
23 



266 REFRACTION AND HOW TO REFRACT. 

Placido's disc shows irregular, distorted circles. 

With Pin-hole Disc.—O. D. V. = ^. O. S. V. = g-. 

With Stenopeic Slit. — Axis 45 degrees before O. D. and 
with +2.25 S., V. =~??. O. S., can not improve 
vision with any glass. 

At Trial-case. — O. D., +2.00 cy^ ax i s 145 ^ ^, ? ?. 
O. S., no glass accepted. 

April 23d : At trial-case O. D., +1.75 cyl. axis 35 = ^-. 

For Mary Smiles. 

R. O. D., — 0.25 sph. 3 +1-75 cyl. axis 35 degrees. 

O. S., plane glass. 
Sig. — Constant use. 

Considerations. — This case shows the advantage of the 
stenopeic slit and the use of the pin-hole disc. The near 
point could not be obtained on account of the poor visual 
qualities and the child's inability to appreciate what was 
wanted. 

Case IX. — Tonic Cramp or Spasm of the Accommo- 
dation. — 

Mrs. L. Age, twenty-four years. 

VI 
O. D. V. =m XL ??. p. p. =9 cm. (?) Add., 24 degrees. Abd., 6 

degrees. Esophoria, 4 degrees. 
O. S. V. = ~~xl~??. P- P- =9 cm. (?) No vertical deviation. 

History of having had glasses changed on three different 
occasions during the past year. Drops were used each 
time, and the three prescriptions were all different. Glasses 
were always satisfactory for the first week, but after this 
time she was always able to see better at a distance with- 
out them. Has pains in her eyes and all over the head 
whenever she attempts to use the eyes with or without any 
glasses. Headaches nearly set her " wild " if she tries to 



APPLIED REFRACTION. 267 

concentrate her vision on a distant or near object. Has not 
been able to read or write or sew for the past two years. 
Has been under the care of the gynecologist and neurologist, 
and they each pronounce her physical condition as normal. 
The neurologist suggests a diagnosis of " hysteria." 
Patient sleeps well and has a good appetite, but will suffer 
from nausea and vomiting if she uses her eyes for any length 
of time. Patient has been married five years. Has one living 
child. No miscarriages. Is apparently in the very best 
of health, and is provoked with her apparent good health 
as not being consistent with her suffering, and hence she 
does not receive any sympathy from her family or her 
friends. 

5. P. — No external manifestations of any ocular irregu- 
larity. 

Manifest Refraction. — 

O. D., — 0.50 S. O -fi.oo cyl. axis 90 degrees = -rpr. 
O. S., the same as O. D. 

Ophthalmoscope. — O. D. and O. S., media clear. Discs 
vertically oval, eye -grounds "woolly." Accommodation 
very active. Shot silk retina. Refraction is compound 
hyperopic astigmatism. 

Cobalt-blue glass shows a red center and broad blue halo. 
(Patient is certainly accommodating.) 

R . Atropin and dark glasses for refraction. 

Static Refraction. — 

O. D. — 1.50 S. = -f I -75 c yl- ax is 9° degrees = -?=-. 
O. S. —1.50 S. = +1.75 cyl. axis 90 degrees = _¥I # 

Patient states that her " pains and headaches all disap- 
peared after using the drops for the third time." 



268 REFRACTION AND HOW TO REFRACT. 

Refraction repeated on three different occasions, and the 
following prescription given : 

For Mrs. L. 

R. O. D., +1.25 S. O + I -75 c yl- ax * s 9° degrees. 
O. S., +1.25 S. O + r -75 c yl- ax i s 9° degrees. 

Glasses properly centered, and accurately adjusted. 

After ten days patient returns with the statement that 
her pains and aches have recurred as before, and that she 
can see better at a distance without her glasses. With 
correction, each eye sees ■— -, and with both eyes can see 
x^x- Add., 20 degrees, and abd., 8 degrees. No verti- 
cal deviation. Has 3 of esophoria at 6 meters. 

R . Atropin -£$ of a grain to the ounce. 

SiG. — To use one drop in each eye each morning and noon. 

To wear a pair of dark glasses with her prescription glasses 
when exposed to any bright light. Not to attempt any 
near-work. This treatment was continued, off and on, for 
six months. Patient was always free from ocular pain and 
headache as long as the atropin was being used, but as 
soon as the ciliary muscle commenced to contract, then the 
pains would return with all their former severity. This 
patient eventually recovered by using her distant correc- 
tion with a pair of plus 2 spheres as hook-fronts for any 
near-work. 

Case X. — Exophoria. — 

Miss. V. B. D. Age, twenty-two years. 

VI 
O. D. V. = -yp p. p. = 0.50 D., type at II cm. 

O. S. V. = -yj-. p. p. = 0.50 D., type at 11 cm. 
Add. and abd., 12 degrees. Exophoria = 4. 

History of seeing double several times a day. Friends 
and members of her family have told her she was " squint- 



APPLIED REFRACTION. 269 

ing." Always returns home with a severe occipital head- 
ache after going shopping or to any place of amusement. 
Has headache when using her eyes, but it soon passes 
away after resting the eyes. 

5". P. — Eyes markedly out under cover. Irises react 
promptly to light, accommodation, and convergence. 
Fixation test shows the right eye divergent. 

Ophtlialmoscope. — O. D. and O. S. No apparent changes, 
and refraction almost emmetropic ; some small amount of 
hyperopia and astigmatism. 

R . Atropin and dark glasses for rest and careful refraction. 

Static refraction, after several repetitions, O. D. and O. 
S., -fo.50 S. ^2 +°-37 c yl- ax ^ s 9° degrees ==-«-. And 
this is ordered, less 0.25. 

With this correction carefully centered, add. =14 degrees 
and abd. = 1 2 degrees, with 3 of exophoria at 6 meters and 
12 degrees of exophoria at 13 inches. This patient was given 
prism exercises for more than two months, and, finally, 
after the adduction reached 30 degrees and abduction was 
10 degrees and 3 degrees of esophoria w T ere obtained, the 
prism exercises were stopped, and patient told to report 
promptly if any discomfort arose at any time. To wear 
her glasses constantly. 

Case XI. — Anisometropia. — 

Mr. Albert S. Age twenty-nine years. In general business. 
O. D. V. = xxx" P- P- type o-5° D. = 25 cm. 
O. S. V. = -Try-, p. p. type 0.75 D. = 30 cm. 
Add., 10 degrees. Abd., 6 degrees. Left Hy., 2 degrees. 

History. — Has had three pairs of glasses ordered, with 
" drops," during the past eighteen months. Has never 
had any but the very slightest relief from ocular pains and 



270 REFRACTION AND HOW TO REFRACT. 

frontal headaches, which have been almost constant for the 
past four years or more. On account of the ocular dis- 
comfort and headaches, the patient has given up all at- 
tempts to read for more than fifteen minutes at a time. 
Patient states that if he uses his eyes for more than this 
length of time they become bloodshot and very tender 
to the touch. General health of patient is excellent ; has 
a good appetite and sleeps well. Does not use tobacco or 
liquor of any kind. 

5. P.- — Face symmetric. Nose very prominent. Inter- 
pupillary distance, 62 mm. 

Ophthalmoscope. — O. D., nerve-head over capillary. Not 
swollen. Accommodation very active. Eye-ground "fluffy." 
Refraction is that of compound hyperopia. O. S., same 
general conditions, but the nerve is vertically oval and the 
refraction is compound hyperopic astigmatism. 

R . Atropin and dark glasses for refraction. 

Static Refraction. — 

O. D., -(-2.00 S. O -f I -°° c yl- ax i s 75 degrees =-yj- ) i e ft 

O. S., +1.25 S. C +3-00 cyl. axis 105 degrees =^j-??? J phoria. 

&. O. D., -f- 1.75 S. O + 1 - 00 cyl. axis 75 degrees O }( /\ base up. 

O. S., +I.00 S. O +3- 00 cyl. axis 105 degrees 3 |^A base down. 
Sig. — For constant use. 

This patient was not made comfortable until he was given 
five -grain doses of the bromid of potash three times a day 
for four weeks. Is now able to use his eyes without the 
least discomfort 



CHAPTER XI. 

PRESBYOPIA.— APHAKIA.— ANISOMETROPIA. 
—SPECTACLES. 

Presbyopia. — The word presbyopia (from the Greek, 
xpiafiuz, " old " ; &$ J "eye ") literally means old sight, and 
patients at the age of forty-five or more years are univer- 
sally recognized as presbyopes, and the condition of their 
eyes as presbyopic. There is no exact age limit as to when 
presbyopia shall begin, the advent of presbyopia being con- 
trolled by the character of the ametropia and physical con- 
dition of the eyes themselves. Presbyopia may be described 
in several different ways, according to the cause — i. e., 

1. Old sight. 

2. The condition of the eyes in which the punctum proxi- 
mum has receded to such a distance that near vision (close 
work) is impossible without the aid of convex lenses. 

3. The condition of the eye in which the lens fibers have 
become more or less sclerotic, and, as a consequence, the 
lens loses some of its inherent quality of becoming more 
convex during contraction of the ciliary muscle. 

4. The condition of the eye in which the power of the 
ciliary muscle has become weakened. 

5. The condition of the eye in which the power of ac- 
commodation is diminished at the same time that the lens 
fibers become sclerotic. 

6. The condition of the eye in which two different refrac- 
tions (not necessarily two pairs of glasses) are required, one 
for distance and one for near vision. 

271 



272 



REFRACTION AND HOW TO REFRACT. 



7. The condition of the eye in which one pair of glasses 
will not answer for distant and also for near vision. 

8. Presbyopia may be described as the condition in which 
nature has instilled a slowly acting but permanent cyclo- 
plegic (the term cycloplegic is used here in a general sense). 

Causes of Presbyopia. — 1. Age. — It is a well-estab- 
lished fact that in childhood the center of the lens begins 
to harden, becomes sclerotic or sclerosed, to form a nucleus ; 
and this process continuing, eventuates in complete sclerosis 
at sixty or seventy-five years. The term sclerosis must not 
be confounded with opacity. 

2. Disease. — Ordinarily, presbyopia, as applied to the 
lens, should be recognized as a physiologic process, as a 
penalty for growing old, though it is a condition which 
may be hastened by disease. Any disease, therefore, which 
will cause the nutrition of the lens to suffer must event- 
ually interfere with its ability to become more convex during 
accommodation. The most common ailments that tend to 
this result are rheumatism, gout, Bright's disease, diabetes, 
lithiasis, la grippe, etc. Any disease which will weaken the 
ciliary muscle will produce presbyopic symptoms. 

Presbyopic Near Points. — The near point and power of 
accommodation in a healthy emmetropic eye, or a healthy 
eye made emmetropic by the addition of correcting lenses, 
is as follows for certain ages : 



Age. 
40 years, 

45 " 

50 « 

55 " 

60 " 

65 " 

70 " 

75 M 



Near Point. 
22 cm. 
28 " 
40 " 

55 " 
100 " 

133 " 

400 " 

00 " 



Power of 
Accommodation. 



4-5° 

3-5o 

2.50 

I.75 or 2.00 

1. 00 

o.75 

0.25 

.00 



diopters. 



PRESBYOPIA. 273 

Ordinarily, the average adult holds a newspaper or book 
at about 33 cm. (13 inches) from his eyes when reading ; 
and if he is forty years of age and emmetropic, or is made 
emmetropic with glasses, he would be using 3 D. of his 
normal 4.50 of accommodation, which would leave a reserve 
power of 1.50 D.; and in this condition, other things being 
equal, he can maintain a reading distance with comfort. In 
fact, he could, by using all of his 4.50 D. of accommoda- 
tion, see objects as close as 22 cm., but not for any great 
length of time, as the ciliary muscle would soon relax. 

This same patient at forty-five or forty-six years of age will 
have lost 1.00 or 1.50 D. of his accommodation, and now 
has only about 3 or 3.50 left ; and if he uses all of it at a 
working distance of 33 cm., the ciliary muscle soon yields. 
In fact, the ciliary muscle can not be held in such a state of 
tension without causing all sorts of pains and aches and 
reflex disturbances ; and the ciliary effort relaxing suddenly, 
the near vision blurs, and the work or reading or sewing 
must be put at a greater distance to obtain relief, or else 
the effort must be abandoned. 

Symptoms of Presbyopia. — The principal symptom is 
that which indicates a recession of the punctum proximum ; 
the patient stating that there is an inability to maintain 
the former reading, writing, or sewing distance, and that all 
near-work must be held at a greater distance than formerly. 
Symptoms of accommodative strain may be present if the 
patient endeavors to force the accommodation to its 
maximum. 

Diagnosis. — The age of the patient and the history of 
having to hold reading matter at an uncomfortable distance ; 
or a history of good distant vision and an inability to retain 
clear near vision — small objects, to be seen, must be held 
far away or "at arm's length." 



274 REFRACTION AND HOW TO REFRACT. 

Correction of Presbyopia. — The presbyopic state repre- 
sents a class of patients for whom glasses may be pre- 
scribed by the manifest refraction, although there are 
exceptional cases in which a quick cycloplegic will be nec- 
essary when an amount of astigmatism or cylinder axis is 
uncertain. 

For a working, reading, writing, or sewing distance of 33 
cm. (13 inches), the writer makes it a rule to add to the dis- 
tance correction at forty-five years of age a -f- 1 sphere ; at 
fifty years of age, a -f- 2 sphere ; at fifty-five years of age, 
a +2.50 sphere ; and for sixty or more years, a +3 sphere. 

The following table for emmetropic eyes shows these addi- 
tions for the different years, and also the near and far points 
with these additions as well as the range of accommo- 
dation or " play " between the near and far points. It will 
be observed that the range of 78 cm. at forty-five years 
rapidly diminishes in the succeeding years, until at sixty 
there is a play of only about 3 inches, and at seventy the 
range is practically gone. 



EARS. 


Add. 


Near Point. 


Far Point. 


Raiv 


45 


+ 1. OO 


22 cm. 


IOO cm. 


78 


5o 


-J- 2. OO 


22 " 


50 « 


28 


55 


+2.50 


23 « 


40 " 


17 


60 


43.OO 


25 « 


33 " 


8 


65 


43.00 


27 " 


33 " 


6 


70 


4-3.00 


30 " 


33 " 


3 


75 


4 300 


33 " 


33 " 






Because a patient is fifty years of age does not signify 
that he will be able to read at 33 cm. with a pair of -f-2 
spheres, or because he is sixty years of age that he can use 
his eyes at 33 cm. comfortably with a pair of +3 spheres ; 
on the contrary, this rule that the writer has given applies 
only to cases of emmetropia. It often happens that pres- 
byopic patients state that they do not want glasses for dis- 



PRESBYOPIA. 275 

tance ; that they do not need them ; that all they wish is a 
pair of glasses to use at near-work, reading, etc. When the 
vision is taken in such cases, it may be found to be — or 
approximating -^=- ; but the young ophthalmologist must 
not be thrown off his guard by this record, as it has already 
been stated that a vision of— does not by any manner 
of means prove the existence of emmetropia. Let the sur- 
geon make it a constant rule in every case of presbyopia to 
ahv ays carefully estimate the amount of the distance ametropia 
first, ?io matter how weak or what its form {sphere or cylinder) ; 
and to the result thus obtained, add the plus sphere which will 
be required for the working distance or point at which the 
patient wishes to see clearly. 

Illustrative Cases. — Case I. — Accepts +0.50 sph. for 
distance. At forty-five years this case would require 
+ 1. 50 sph. for reading at a distance of 33 cm.; at fifty years, 
-[-2.50 sph.; at fifty-five years, +3 sph.; and at sixty or 
more years, +3.50 sph. Only one pair of glasses is neces- 
sary. 

Case II. — Accepts -j-2 sph. for distance; at forty-five 
years these eyes would require -f- 3 sph.; at fifty years they 
would require +4.50 sph.; and at sixty or more years they 
would require +5 sph. Two pairs of glasses would be 
indicated in this case. 

Case III. — Accepts — 1.00 sph. for distance ; at forty- 
five years this patient could read without any glasses, as 
— 1 for distance would be neutralized by the -f I required 
for reading. At fifty years, however, the patient would 
require a + 1 sphere for near, and at fifty -five a -f- 1.50, and 
at sixty years a -\-2 sphere. Case II required two correc- 
tions, one for distance and one for near ; and the same may 
be said about Case III ; but in this latter instance there was 
a time at forty-five years when there was no necessity for 



276 REFRACTION AND HOW TO REFRACT. 

glasses for the near-work, as the patient's eyes were in a 
suitable condition of refraction to read without them. 

Case IV. — Accepts — 3 sph. for distance. At forty-five 
years would require — 2 sph. for reading; at fifty years 
would require — 1 sph. for reading ; and at sixty years 
can read without any glasses. Such a patient says he has 
gotten his " second sight." 

Case V. — Accepts -f-0.50 cylinder axis 180 for distance 
and requires the usual additional spheres for the increasing 
years for his reading distance. 

Case VI. — Accepts -f-1.00 sph. O -}-i-00 cyl. axis 180 
for distance and requires the spheric additions as the years 
increase. Two pairs of glasses should be prescribed. 

Case VII. — Accepts — 1 cyl. axis 90 for distance, and 
requires -f 1 cyl. axis 1 80 to read with at forty-five years 
of age ; at fifty years he requires -J- 1 sph. O -f- 1 cyl. 
axis 180 ; and at sixty years requires -f- 2 sph. O + 1 cyl. 
axis 180. At forty-five years of age this patient is com- 
monly spoken of as having simple myopic astigmatism for 
distance (against the rule) and simple hyperopic astigmatism 
for near (against the rule also) ; two pairs of glasses are in- 
dicated throughout life. 

Case VIII. — Accepts — 1.00 sph. O — 1.50 cyl. axis 
1 80 for distance, and atforty-five years will need — 1.50 cyl. 
axis 180 for reading ; at fifty years will require + 1. 00 sph. 
O — 1.50 cyl, axis 180 degrees ; at sixty years, +0.50 
sph. O +1.50 cyl. axis 90 degrees. 

Two pairs of glasses should be used throughout life. 
At forty-five years this patient has a compound myopic 
correction for distance and simple myopic astigmatism for 
near ; at fifty years the correction for near is that of crossed 
cylinders (mixed astigmatism) ; and at sixty years the near 
correction is that for compound hyperopic astigmatism. 



PRESBYOPIA. 277 

Case IX. — Accepts — 1 .00 sph. O -\-2 cyl. axis 90 for dis- 
tance (mixed astigmatism) ; at forty-five years, + 2 cyl. axis 
90 is required for reading; at fifty years, -f-J-OO sph. O 
+ 2.00 cyl. axis 90. Two pairs of glasses are required. At 
forty-five years the distance correction is for mixed astig- 
matism and the reading correction is for simple hyperopic 
astigmatism. 

Case X. — Accepts — 2.00 cyl. axis 180 for distance ; 
at forty-five years requires a mixed astigmatism correction 
for near ; at fifty years, a simple hyperopic correction ; 
and a compound hyperopic correction at sixty years. 

In the above illustrative cases the working distance has 
been calculated at 33 cm., or 13 inches; but as some 
patients use their eyes at a greater or less distance than 
this, the additional convex lenses must be calculated accord- 
ingly. For instance, the weaver at fifty-five years of 
age who requires +2 spheres for distance could not see to 
weave at 50 cm. ; if +2.50 spheres were added to his dis- 
tance correction, all he needs is -j- 3 for his working dis- 
tance. Or the diamond cutter who wishes, glasses to see 
his work at 8 inches, if he accepted — 1.00 sph. for distance, 
he would require -j- 2 s P n - a t forty-five years of age. 

In conclusion, there are three facts in the refraction of 
presbyopic patients that should receive attention : 

1. Many accept a weak plus cylinder (-f-0.50 at axis 
180) against the rule. This is presumptive evidence that 
the astigmatism is acquired, is lenticular, and is due to the 
sclerotic changes previously mentioned. The only positive 
way to prove this fact is by the retinoscope, and by the ab- 
sence of corneal astigmatism with the ophthalmometer. 
If the case has been previously refracted by the same sur- 
geon, his record will also confirm this extremely interesting 
occurrence. According to able authorities, hyperopic eyes 



278 REFRACTION AND HOW TO REFRACT. 

become more hyperopic after the age of seventy years, 
and emmetropic eyes may become hyperopic, and myopic 
eyes less myopic, from the same sclerotic or shrinking pro- 
cess which takes place in all the ocular tissues as a result 
of senility. The method of correction by glasses, however, 
is just the same, and that is to correct the distant vision 
first and then add the near correction. 

2. An attack of glaucoma may precipitate presbyopic 
symptoms, so that when a presbyopic patient asks for fre- 
quent changes in his corrections, this complication should 
be borne in mind. 

3. The swelling of the lens which occasionally precedes 
the formation of some forms of cataract should be remem- 
bered when the patient develops symptoms of myopia — i. e., 
a reduction in the strength of convex glasses. 

Aphakia (d, priv. ; <pax6<; " lentil") literally means an 
eye " without a lens." (See Fig. 174.) An eye which has 

had its lens dislocated 

has been erroneously 

spoken of as aphakic. 

The absence of the lens 

means a total absence 

of all accommodation, 

FlG I7 " no matter what the age 

of the patient may be» 

Causes. — Aphakia may be congenital, but in most cases 

is the result of removing the lens by operation. 

Diagnosis. — Aphakia maybe diagnosed by inspection — 
i. e. y corneal scar, depth of anterior chamber, tremulous 
iris, coloboma of the iris, opaque capsule whole or in part, 
erect corneal image, with absence of lenticular images, and 
by the patient's history. 

The ametropia of an aphakic eye depends in great part 




HETEROMETROPIA. 279 

upon the previous refractive condition of the eye, and also 
upon the kind of operation that was performed for the re- 
moval of the lens. It has been calculated that an eye, to 
be emmetropic after the removal of its lens, would have to 
be myopic at least twelve diopters. If this is always true, 
then the correcting lens which is selected by an aphakic eye 
is a guide to its former ametropia. An eye which selects a 
weak plus sphere would, therefore, have been myopic before 
the operation ; and if about a + 12 S., its previous refrac- 
tion approximated emmetropia ; if a plus sphere stronger 
than 12, then the previous refraction was very likely hyper- 
opia. 

An eye which has had its lens removed by absorption 
(needling) is not likely to be astigmatic ; whereas, when 
the lens has been removed by extraction, astigmatism 
against the rule of one or more diopters almost invariably 
results, and the axis of the correcting cylinder generally 
coincides with the points of puncture and counterpuncture 
in the cornea. If a patient had 2 or 3 D. of myopic 
astigmatism with the rule, this would be neutralized by the 
corneal section. 

Correction of Aphakia. — As in presbyopia, two correc- 
tions are necessary — one for distance and one for near. 
Astigmatism must always be looked for and carefully cor- 
rected, especially if the lens has been removed by extrac- 
tion. 

Case I. — 

vi 

O. D., 48.00 sph. 3 -l-S- 00 c yl- ax i s IO degrees. V. = -jg— 

O. D., 4 11.00 sph. 3 ■+ 3-°° cyl. axis 10 degrees = reading at 33 cm. 

This patient was presumably myopic before operation. 
Heterometropia {irepo<; t ''different"; t*erpov f "a meas- 
ure"; <»</', "the eye") literally means that the ametropia of 



280 REFRACTION AND HOW TO REFRACT. 

the two eyes is different in character; examples, O. D. -f- 1 D. 
and O. S. — I D., or O. D. +3 cyl. axis 90 and O. S. — 3 
cyl. axis 180 , or O. D. — 5 D. and O. S. — 5 cyl. axis 
180 , etc. 

Anisometropia («Wo?, "unequal"; fiirpov ) "a measure"; 
&4\ "the eye") literally means that the ametropia of the 
two eyes is the same in character but of unequal amount. 
Examples, O. D. +2 D. and O. S. +6 D., or O. D. —0.50 
D. and O. S. —5 D., or O. D. +3 cyl. axis 90 and O. S. 
-f-6 cyl. axis 90 , etc. This condition may be slight or 
one of the most extreme conditions imaginable. 

For instance, if both eyes have compound hyperopic 
astigmatism, they are not considered as heterometropic, even 
if the sphere and cylinder are of different strength in the 
two eyes. Bearing this distinction in mind, the percentages 
already given for myopia, hyperopia, the different astigma- 
tisms, etc., have been calculated accordingly, that for 
heterometropia being about thirteen per cent. 

Causes. — Usually the condition is .congenital, or it may 
be acquired. 

Difficulties. — Two difficulties are encountered when or- 
dering glasses for cases of anisometropia or heterometropia: 
(1) The lens for one eye may be concave and that for the 
other may be convex, or both eyes may require a convex 
or both may require a concave lens, but one very much 
stronger than the other; under these circumstances, when 
the eyes are rotated there will be a prismatic result of dif- 
ferent amount in each eye, and this may mean diplopia, or 
at least an exertion on the part of the extraocular muscles 
to prevent diplopia which will cause dizziness, nausea, head- 
ache, etc. (2) With lenses as just mentioned, the size of 
the two retinal images will not be exactly the same, and this 
will mean an interference with clear binocular vision. 



ANISOMETROPIA. 28 1 

For purposes of study, the writer would divide cases of 
heterometropia and anisometropia into four different classes. 

Class I. — This class embraces those cases in which the 
difference in the ametropia between the two eyes is very 
slight or does not exceed two diopters. In fact, there are 
very few pairs of eyes that are not slightly anisometropic ; 
such eyes usually receive their exact corrections w r ith com- 
fort, regardless of the condition. 

Class II. — Cases that come under this head also accept 
their exact correction for each eye, but do not attempt 
binocular fixation, and may never suffer the least in- 
convenience; these cases are extremely rare. They do not 
complain of diplopia, as they have learned to ignore the 
false image. Cases of alternating squint, one eye myopic 
and the other eye hyperopic, may be included in this class. 

Class III. — This is a class which will accept the exact 
correction before one eye only, and the eye which has the 
greatest amount of ametropia will refuse almost any lens 
except the very weakest. The eye that has the most ame- 
tropia is often quite amblyopic. 

Class IV. — This class includes young children especi- 
ally ; cases of squint. In children the correction as found 
by the static refraction is usually accepted. 

The Prescribing of Glasses in Cases of Heterometropia 
and Anisometropia. — Excluding Class I, there is no fixed 
rule to follow when ordering glasses in decided cases of 
heterometropia or anisometropia, and, in fact, such eyes are 
a constant study to the most able ophthalmologist. The 
younger the patient, however, the more likelihood of a favor- 
able result from the careful selection of a glass for each eye ; 
but when the patient is an adult, it becomes a very serious 
question as to what glass to prescribe that will give satisfac- 
tion. As good results are to be expected in children, they 
24 



282 REFRACTION AND HOW TO REFRACT. 

should receive the most careful retinoscopic refraction. The 
child comes under observation on account of a squint, and an 
operation for the deformity is often demanded ; but the opera- 
tion must be refused until the ametropia has been carefully 
treated. Glasses having been prescribed, the squinting eye is 
put to work to develop its seeing qualities, which have been 
permitted to lie dormant for want of a proper glass. To do 
this, the "good" eye is shielded or blinded with a hand- 
kerchief tied over it, or a blinder (see Fig. 1 60) placed over 
its correcting lens, for an hour or two each day, and in this 
way an attempt is made to bring the vision in the squinting 
eye up to that of its fellow. 

Or another way to develop the vision in the squinting 
eye is to use a cycloplegic in the " good " eye, so that the 
squinting eye must do most of the work. This is rather 
trying to the little patient, and often means the additional 
use of dark glasses. As a rule, the "good" eye has the 
least amount of ametropia, but occasionally the reverse con- 
dition may exist. 

In a case like the following, the little girl, five years of 
age, was brought on account of convergent squint in O. S., 
which developed or commenced to appear when ten months 
of age, and the parents attributed it to the habit of sucking 
her thumb at the time of being weaned. Refraction, with 
atropin as the cycloplegic, and obtained with the retino- 
scope, showed O. D., -(-2.00 sph.; O. S., +4.00 sph. O 
+ I.OO cyl. axis 75 degrees. 

This child developed the squint on account of the 
monocular astigmatism and because it could not accom- 
modate sufficiently with the left eye. To avoid diplopia 
at the same time that the eyes were converging, the 
left eye naturally turned inward. With correcting glasses, 
and practising as above directed, the squint entirely 



BIFOCALS. 283 

disappeared, and vision one year later was — in each 
eve with the correcting glasses. If the glasses are laid 
aside for any length of time, the squint returns. This 
child must wear the glasses or have "squint." 

To make sure that no injustice is done to an apparently 
amblyopic eye in an adult (Class III, p. 270) where ambly- 
opia exanopsia has existed for many years and nothing has 
been done to improve its correction, the writer makes it a 
rule to prescribe the exact correction for each eye, and at the 
time of ordering the glasses explains to the patient what 
the purpose and desire is, and that if there is any great 
amount of discomfort in any way, he must return and have 
any necessary change made in the glass. These patients 
should be kept under observation and the amblyopic eye 
given some sort of a correction and improved as much as 
possible ; the purpose being not to allow the eye to 
degenerate or grow more amblyopic, for if any accident 
should befall the " good" eye, then the amblyopic eye will 
often be a friend indeed. 

Glasses for Presbyopes and Cases of Aphakia. — Unless 
the distant vision is improved or asthenopic symptoms are 
relieved by glasses, it will be sufficient to prescribe the 
near correction only. When a distant and near correc- 
tion are required, they may be prescribed as two pairs of 
glasses in separate frames, or two pairs in one frame, known 
as bifocals. Bifocals, or what is equivalent to bifocals, are 
made in different ways. 

1. Franklin * or Split Bifocals (Figs. 175 and 176). — 
This form of bifocals consists of an upper and a lower lens, 
each with its individual center ; the upper lens is for dis- 
tance and the lower for near vision. Such lenses must have 
the frame all around the edges, so as to hold them in posi- 

*" History of Spectacles," L. Webster Fox, " Med. and Surg. Reporter," 
1890, vol. lxif, 513-519. 



284 



REFRACTION AND HOW TO REFRACT. 



tion. Bifocals of this kind are not in common use. The 
field of distant vision is limited by the unnecessarily large 
near correction, and where the two lenses come together, 




Fig. 175 



Fig. 176. 



there is apt to develop chromatic aberration and a decided 
prismatic effect when the vision is directed through this 
space. 



K. 



SlG.- 



O. D., -j- 2.00 sph. 
O. S., + 2.00 sph. 
—For distance. 



R. O. D., -f 4. 00 sph. 
O. S., +4.00 sph. 
Sig. — For near. 
Directions to Optician. — Make into Franklin or split bifocals. 

2. Morck's Patent or " Perfection" Bifocals (Fig. 

177). — These are a modification of the Franklin or split 

bifocals, and in place of having 
lenses united in a horizontal line, 
the near and distant lenses are 
fitted together with correspond- 
ing crescent edges. This form 
of bifocal gives a larger field for 
the distance correction, and, like 
the Franklin, is much better 
for those who work in a damp 

atmosphere and can not wear the cement bifocal. It is, 




BIFOCALS. 



285 



however, more expensive than the cement form ; but, like 
the Franklin, it often looks clumsy or heavy on account 
of the frame. "Perfection" or "Morck" bifocal must be 
signified in writing the prescription. 

3. Cement Bifocals (see Figs. 178 and 179*). — This is 
the most common form of bifocal and the least expensive 
in its original cost, as also when making changes in the 
near correction. This bifocal is made by cementing a seg- 

A 




Fig. 178. 



Ill 




Fig. 179. 




Fig. 180. 




Fig. 1S1. 



Fig. 182. 



ment of a small periscopic sphere on to the lower part of 
the distance correction. This periscopic sphere or disc or 
segment, as it is called, has a prismatic quality (see Fig. 
179) suitable to the exigencies of the individual lens to 
which it is cemented. The segment may be of any shape 
desired. Those in common use are shown in figures 180, 
181. and 182. It is cemented to the distance correction 



* Described by Dr. Geo. M. Gould, "Med. and Surg. Reporter," Nov. 3, 
1888. 



286 



REFRACTION AND HOW TO REFRACT. 



with Canada balsam. While this is the usual method of 
making a cement bifocal, yet it may be made by cementing 




Fig. 183 



a concave segment to the upper part of the near correction. 
(Figs. 183 and 184.) This form is not in common use. 

R. O. D.,+2S. 

O. S., +2 s. 
Cement on the lower part of the above O. D. and O. S., -|-2.oo S. 
SlG. — Make frameless bifocals. 

Or, 

R. O. D., + 4 S. 

o. s., + 4 s. 

Cement on the upper part of O. D. and O. S., — 2.00 S. 
SlG. — Make frameless bifocals. , 

4. Achromatic Bifocals (Figs. 185 and 186*). — This 



Fig. 185. 



W 
Fig. 186. 



form of bifocal is used principally in cases of aphakia where 
the plus sphere is quite thick and correspondingly heavy. It 



* Borsch patent. 



BIFOCALS. 



287 



is made in one of two ways : (1) By grinding out a portion 
of the lower part of the distance correction (in crown glass) 
and cementing into the concavity a biconvex segment 
of flint glass. This form of bifocal is a combination of 
the "perfection " and lenticular. (2) In place of grinding 
the concavity in one lens, as just described, this achromatic 
bifocal is also made by taking two planoconvex spheres 
and grinding out a concavity in each, and then inserting a 
convex sphere of flint glass, as shown in figure 186 ; these 
three lenses are then cemented together, and when com- 
pleted, look like the cement bifocal, as shown in figure 182. 
It is a matter for very careful calculation as to just how 
strong to make the flint glass segment, so that the result 
may be just exactly right. The merits of this bifocal are 
lightness and the absence of chromatic aberration. These 
lenses are very expensive. 

5. Solid or Ground Bifocals (Figs. 187 and 188). — 
Lenses of this character are made in one piece by grinding 



V 

Fig. 187. 



Fig. 188. 



on to the upper part of the near correction the necessary minus 
spheric correction for distance. They look neat, but are not 
always comfortable, on account of the resulting prismatic 
effect, which is especially apt to occur when the lens is con- 
vex, though this may not be so troublesome a feature when 
the lens is moderately .concave. 



288 



REFRACTION AND HOW TO REFRACT. 



Single Crystal Bifocals. — Lenses of this character are 
made in one piece, and in this respect are like the solid 
bifocal, but they differ from the solid bifocal in two ways : 
the near correction is made by grinding on to the lower 
part of the distance correction, the necessary plus sphere. 
The solid bifocal gives a decided prismatic effect where the 





Fig. 189 



Fig. 190. 





Fig. 191. 



Fig. 192. 




Fig. 193 



two corrections meet, whereas this defect is said not to 
exist in the crystal bifocals. These bifocals are expensive. 
6. Patients who have a very weak distance correction, 
and could do without it, sometimes accept it for the con- 
venience of wearing bifocals ; they do not wish to be an- 
noyed by taking off or putting on a near correction, prefer- 



BIFOCALS. 289 

ring to have the glasses where they can find them; business 
men especially. Other patients prefer to do without a dis- 
tance correction, and will often use a near correction that 
has one-third or nearly one-half of its upper part cut away, 
so that they can look over the near correction when they 
wish to see at a distance. (See Figs. 189, 190, 191, 192, 
and 193.) Myopes who do not need a near correction will 
wear their distance correction with its lower portion cut 
away, so that when they wish to see near at hand, they can 
look under the distance correction. 

7. Patients who require a distance correction, and can 
not get accustomed to cement segments, and at the same 
time do not wish to change the distance correction, but 
prefer to keep it on all the time, can put on their addition 
for near vision in the form of hook or "grab" fronts of the 
same size as the distance lenses or reduced one-half in the 
vertical diameter. This is not always a good combination, 
as in every instance the lenses do not lie in contact with 
each other. 

8. Lorgnettes may be used as a distance correction or as 
a substitute for hook fronts. Some myopic women who 
wear their near corrections constantly often carry lor- 
gnettes, which they hold up in front of the near correction 
to improve distant vision for a few minutes, or, wearing the 
distance correction, can use a plus lens in the lorgnettes for 
near vision. 

9. Cases of monocular aphakia where the vision in the 
fellow-eye is very defective can wear reversible frames, one 
lens for distance and the other for near, — that is to say, a 
frame which has a free joint at the temples, — and in this way 
they avoid bifocals, and can change the distance for the 
near correction, by turning the temple-pieces. 

In some cases of aphakia where the lens is very powerful, 
25 



29O REFRACTION AND HOW TO REFRACT. 

a bifocal segment can sometimes be dispensed with, if the 
patient has a long nose, by sliding the lens down from the 
eye and then holding the reading matter at the conjugate 
focus. A toric lens (Fig. 194) is very acceptable in occa- 
sional instances, as it reduces somewhat the weight and 
thickness of the lens, and also enlarges the field of vision. 
A toric (torcine or torique, "twisted") lens is one which 



1 / ji 



Copyright, 1886, by Chas. F. Prentice. 
Fig. 194. 

has, combined in one surface, the optic effects of a sphero- 
cylindric lens, or two cylinders of different strength at right 
angles to each other. Unfortunately, this form of a lens is 
quite expensive. 

General Considerations. — Before prescribing any pair 
of glasses, the patient should have the opportunity to wear 
the correction in the office for a short time, that he may 
study its effect; this is especially necessary (i) when the 
glasses are strong ones ; (2) when there is monocular as- 
tigmatism ; (3) when one lens is much stronger than the 
other (anisometropia) ; (4) when the astigmatism is asym- 
metric ; or (5) when there is a strabismus, etc. The patient 
loses confidence (and the surgeon is not made happy) when 
the patient returns with his glasses in his hands and states 
that he can not wear them — that they make him " dizzy " or 
" tipsy " ; that the glasses make the pavement, houses, trees, 



BIFOCALS. 29I 

people, pictures on the wall, chairs, tables, etc., all appear 
as if they were going to fall to one side. The surgeon 
should have anticipated all this, and assured the patient 
beforehand that, after a little perseverance and practice, this 
distortion (parallax) will disappear ; and if not, then a 
change will have to be made in the glasses. Very often 
the whole difficulty is due to a want of proper centering of 
the lenses, presuming, of course, that the glasses ordered 
are perfectly correct. 

Patients who require weak lenses — spherocylinders or 
cylinders alone — may at some time be informed that " the 
correction is but window-glass," and thus the surgeon may 
be put in disgrace as having prescribed for mercenary 
reasons, when in truth the glasses have already cured an 
old blepharitis or asthenopia. In ordering weak correc- 
tions, therefore, the character and purpose of the glasses 
should be imparted to the patient. 

It is interesting to notice that strong glasses are usually 
ordered to improve the vision, and not always for the relief 
of asthenopia, whereas weak corrections are prescribed for 
the relief of headaches, etc., without any decided improve- 
ment in the vision which the patient can appreciate when 
looking at a distance, and many such patients will say they 
can see just as well without their glasses. When strong 
plus spheres are prescribed for a child, it will do no harm 
to inform the parents of the character of the glasses, so 
that when a presbyope tries the child's glasses, the sur- 
geon may not be accused of ruining the child's eyes by 
having ordered a pair of glasses strong enough for a grand- 
mother to read with, and the child hurried off to a rival 
confrere to have the " outrage " rectified. 

A patient who has fought against the inevitable, using 
headache powders, liver pills, etc., in the vain hope of not 



292 REFRACTION AND HOW TO REFRACT. 

having to put on glasses, may still object to their use for 
various reasons. It may be that glasses will not add to 
the personal appearance, or the parents may dislike the 
idea, fearing that " the oculist puts glasses on every 
patient," or that " the eyes will never be the same again," 
or that " the habit of wearing glasses, once established, can 
never be stopped." These and many other statements will 
serve to enliven the daily routine of ophthalmic practice. 
These objections having been met from the point of view 
of the patient's individual welfare and future good of his 
eyes, the next question that arises is what form of glasses 
shall be prescribed. 

Spectacles. — The child is certainly a candidate for spec- 
tacles. The frames must be very durable, and preferably 
of 14-carat gold. Spectacle frames keep the lenses in posi- 
tion, and the lenses are then less liable to be broken than 
in the form of eye-glasses, and for most occupations are to 
be preferred. Occasionally, the shape of the nose will pre- 
clude the use of anything else but spectacles. When one 
lens is very heavy or both have considerable weight, or 
when one or both lenses are cylindric, with axes inclined, 
spectacles are certainly indicated. 

Eye-glasses, also called "pinc-nez," are for the adult, 
and may be prescribed when the lenses are not too heavy, 
or the cylinders too strong or their axes inclined. Eye- 
glasses are easily bent, and lose their exact positions before 
the eyes. For the young society girl nothing but the most 
delicately made eye-glasses will, as a rule, be accepted. 

Bifocals. — These should not, as a rule, be prescribed if 
the lenses are very strong or the correction a complicated 
one, or the patient advanced in years and has never at- 
tempted them before, or if the patient is very portly or 
uncertain in his gait, or the vision is not brought close to 



BIFOCALS. 293 

the normal. Two separate pairs of glasses are to be recom- 
mended under these circumstances. When ordering any 
pair of bifocals, the patient should be cautioned and in- 
structed that when looking downward, going up or down 
stairs, getting into or out of a conveyance, he is to look to 
one side or over the segment of the bifocal and not 
through it, otherwise he will be liable to make a false step 
or misjudge the distance, which might mean serious bodily 
injury, for which the surgeon does not wish to hold himself 
responsible. 

Glasses for constant use should be placed perpendicularly 
or at an axis of about 5 degrees to the plane of the face, 
with the optic centers corresponding to the pupillary cen- 
ters when the eyes are directed to a distance. If the lenses 
are unusually strong and to be used principally at near- 
work, then it may be necessary to consider the advisability 
of having two pairs of glasses, one for distance and one 
for near, each with the centers to answer for the object in 
view. If only one pair of glasses has been ordered, and 
they happen to be very strong, then a pair of prisms in 
hook fronts may have to be used at the near-work, so as to 
counteract the prismatic effect of looking through the dis- 
tance glasses during convergence. Glasses for near-work 
only should be put into a frame made especially for the 
purpose, so that the lenses may have an inclination in 
keeping with the downward turn of the eyes, and thus be 
perpendicular to the axis of the eyes, and the lenses should 
be decentered inward to equal the convergence. The one 
serious objection to bifocals in certain instances is that the 
glasses can not be made with the inclination suitable for 
both distance and near vision, and very often there must be 
a compromise between the two. 

The surgeon should make it a point to carefully inspect 



294 REFRACTION AND HOW TO REFRACT. 

every pair of glasses which he orders, as his painstaking 
efforts and best endeavors may be completely frustrated by 
poorly fitting lenses. 

1. The lenses should neutralize. (See p. 58.) 

2. The optic centers should be at the points indicated. 

3. The cylinder axes must be exact. 

4. The lenses must be perpendicular or inclined to the 
front of the eye, as necessary. 

5. The distance of the lenses from the eyes should 
always be sufficient to clear the lashes ; and if these are 
very long, they may have to be trimmed. 

6. The most convex or the least concave surface of the 
lens should be placed away from the eyes. Or the most 
concave surface toward the eye. 

7. The lenses should be of the correct size for the indi- 
vidual face. These and many other points for the average 
case must receive the careful consideration of the surgeon. 

Tinted or Colored Glasses. — Except for the relief of 
photophobia following cataract extraction, mydriasis, or 
inflammatory 'diseases, the surgeon does not order colored 
glasses. Colored lenses are to be deprecated except in the 
cases just mentioned, as they only increase the tendency to 
photophobia instead of correcting it. 

Perimetric Lenses. — These are made to conform in out- 
line to the normal field of vision as recorded by the per- 
imeter, hence the name.* 

The usefulness of the perimetric lens is limited to those 
cases in which the correction contains a plus cylinder and 
the lens is of moderate strength. It is not a lens that can 



* The writer described this form of lens before the Section in Ophthalmology 
of the College of Physicians of Philadelphia, in March, 1897. 



TRIFOCALS. 



295 



be prescribed in myopia or aphakia. The purpose of the 
perimetric lens is to give a normal field and have the edge 
of the lens sufficiently removed that the patient may not be 
disturbed by seeing it. It certainly enlarges the field of 
vision, and in this way is a great advantage in certain occu- 
pations, playing the piano, etc. Figure 203 or 204 may 
answer the same purpose if properly centered. 

Trifocals. — Occasionally, a patient is not content with 
bifocals, but will demand a focal point somewhere between 
infinity and his working distance ; this can only be produced 
by cementing two segments of different sizes and strength 




Fig. 195. 




on the intermediate correction. (Figs. 195 and 196.) Book- 
keepers who have to work at large and lengthy ledgers find 
great comfort in this combination, though to be of special 
service the lenses must be made large. Example: +2.00 
equals working distance at I meter. Minus I diopter 
added above equals infinity vision. -f2.oo added below 
gives near vision at 1 3 inches. 

Decentering of Lenses. — Instead of writing a prescrip- 
tion for a lens and prism, the prismatic effect of the lens 
may be obtained by decentering the lens. The rule is 



296 REFRACTION AND HOW TO REFRACT. 

that for every centimeter of decentering there will result 
just as many prism-diopters as there are diopters in the 
meridian of the correcting lens. For example, +4 sph. O 
4 P. D., base out, is the same as +4 sph. decentered I cm. 
outward; or +4 sph. O 2 P. D., base in, equals +4 S. 
decentered 5 mm. inward ; or -(-2 sph. O +2 cyl. axis 
90 degrees O 2 A, base outward, equals -j- 2 sph. O -f 2 
cyl. axis 90, decentered 5 mm. outward. 

While it is well for the student to know how to decenter 
lenses, yet the writer does not recommend such lenses, 
preferring, when necessary, to order a prismatic combina- 
tion, and have the optician fill the prescription, starting 
direct from the prism. 



CHAPTER XII. 

LENSES, SPECTACLES, AND EYE-GLASS 
FRAMES. HOW TO TAKE MEASURE- 
MENTS FOR THEM AND HOW THEY 
SHOULD BE FITTED. 

The selection of the size and shape of lenses, the char- 
acter of the spectacle and eye-glass frames and their adjust- 
ment, is the work of the optician. It occasionally happens, 
however, that the surgeon may not have an optician in his 
town, and will, therefore, have to take the necessary meas- 
urements himself and send them, with his prescription, to 
an optician in a neighboring city. This chapter is there- 
fore added for the benefit of such surgeons. It is hardly 
necessary to state that the frames should be very carefully 
adjusted and the lenses centered to the patient's eyes. A 
lens improperly adjusted may utterly destroy the good 
effect of the most skilfully selected correction, giving dis- 
comfort to the patient and reflecting seriously upon the 
surgeon's ability. In fact, it is always well for the surgeon 
to personally inspect every pair of glasses which he may 
order. 

Lenses. — These are spoken of as "eyes," and come in 
various sizes and shapes. They are spoken of as O, double 
O (00), triple O (000), etc. (See Figs. 202, 203, 204, 
205, and 206.) Or sizes smaller than O are numbered 1, 2, 
3, or 4. (See Figs. 197, 198, 199, 200.) Different shapes 
and sizes are lettered A, B, C, D, F, or X. (See Figs. 189, 
190, 191, 192, 193, 201.) All these lenses are also marked 

297 



290 REFRACTION AND HOW TO REFRACT. 

in millimeters of breadth and length. The lenses for indi- 
vidual patients are selected according to the purpose for 
which they are intended, and particularly to be in keeping 





Fig. 197. 



Fig. 198. 





Fig. 199. 



Fig. 200. 





Fig. 201. 



Fig. 202. 



with the facial measurements. The size or " eye " O (39 X 30 
mm.) is the usual size for the average adult, and number 
2, 3, or 4 is for a child. C, D, or F may be ordered for a 



LENSES. 



299 



presbyope who does not need a distance glass and who 
does not wish to be taking - off the near correction to see at 




Fig. 205. 




Fig. 206. 



a distance ; in other words, such a shaped lens can be 
looked over without any difficulty. Or the presbyope who 



300 REFRACTION AND HOW TO REFRACT. 

requires a — 2 for distance and can see to read without 
any near correction, — being about fifty years of age, — could 
have his minus lenses made in the shape of A, B, C, or D 
inverted, and, wearing this for distance, would look under 
it when he wished to see near at hand. As a rule, the 
patient with a narrow face and short interpupillary distance 
will require a small "eye," whereas the patient with a 
broad face and long interpupillary distance will require a 
large "eye." 

Spectacle Frames (Fig. 211). — These consist of a nose- 
piece (called the bridge) and temples (called sides). These 
are attached to the lenses ("eyes") by screws passing 
through holes which have been drilled through them, mak- 
ing what is known as the frameless spectacles ; or a wire is 
fitted around the lenses, to which the bridge and sides are 
attached with solder, forming the "framed" spectacles. 

Eye-glass Frames (Fig. 212). — These consist of a 
spring and nose-pieces ; the latter are called guards. 
Framed and frameless eye-glasses have the nose-pieces or 
guards attached to the lenses as in the spectacles. 

How to Take Measurements. — There are three points 
that require particular attention : (1) The center of the 
lens should correspond with the center of the pupil ; (2) 
the lens must be just far enough from the eyes to avoid the 
lashes, and if these are very long, they must be trimmed ; 
(3) the lens must be at such an angle that the visual axis 
will be perpendicular to it. 

First Measurement. — The Interpupillary Distance.- — To 
accurately measure the distance from the center of one 
pupil to the center of the other is not always an easy thing 
to do, especially if the pupils are dilated ; hence, it is good 
practice to measure this distance from the inner side or edge 
of one pupil to the outer edge of the other. This measure- 



FIRST MEASUREMENT. 



301 



ment can be made with an ordinary rule divided to six- 
teenths of an inch or in millimeters, or with a special instru- 
ment for the purpose, called a pupilometer. The patient 
is told to look directly to the front, at an object across the 
room, and the surgeon, in front, with his head nearly in the 
line of sight, holds the rule across the patient's face, as 
close as the bridge of the nose or eyelashes will permit. 
With his thumb-nail as a marker, the surgeon gages the 
distance as indicated (see Fig. 207), which illustrates the 
conditions. In taking this measurement the surgeon should 
be at an arm's length from the eyes, for the reason that his 




Fig. 207. 



own eye forms the apex of a triangle of which the eyes of 
the patient form the base, and the measurement is apt to 
be two or three or four millimeters short if he gets too 
close. 

If the glasses are to be worn for distance only, then the 
measurement must be for the full interpupillary distance, as 
the patient looks into infinity ; but if the glasses are for near- 
work only, then the distance between the pupils must be 
correspondingly diminished, and the measurement taken 
as the patient looks at a near point. If the glasses are 
to be worn for both near and far vision, for constant use, 



302 



REFRACTION AND HOW TO REFRACT. 



then the center of the lenses must be placed intermediate 
between the distance and near measurements. 

Second Measurement. — The Bridge. — The regulation 
spectacle bridge is known as the saddle-bridge, and should 
conform to the exact shape of the patient's nose. It is in- 
tended to remain in just one place, and that is at the bridge 
of the nose (see B in Figs. 208 and 209), the place where 
the nose begins to extend outward after passing down from 
the forehead. The points B and D, as shown in figure 210, 
represent the widest part or base of the bridge. A and R 
are the arms, which extend upward or outward and are 
fastened to the lenses. The length of the arms controls in 





Fig. 208. 



Fig. 209. 



great part the distance of the lenses from the eyes. To 
raise or lower the position of the lenses in front of the eyes, 
the posts or arms alone should be bent ; the bridge itself 
should never be tilted, as its edge will cut into the skin of 
the nose ; this is a most important consideration for the 
patient's comfort. 

The Shape and Size of the Bridge.— To take this meas- 
urement, the surgeon should have a piece of lead-wire or 
thin, pliable copper-wire ; the lead-wire is best. This wire 
is accurately molded to the bridge of the patient's nose, 
the arms (A and R) are bent to the proper angle, and then 
the ends of the wire are curved or bent outward to show 
the plane of the lenses. (See Fig. 210.) 



THIRD MEASUREMENT FOURTH MEASUREMENT. 303 

When the wire has been bent into place and the eyelashes 
do not touch at L and L, it is removed and placed on the 
under surface of a piece of paper, when an impression and 
lead-pencil tracing is made of it. If the measurement is 
not taken in this way, then the surgeon, with a pair of 
moderately blunt-pointed compasses, measures the breadth 
of the nose from B to D, and also the height of the bridge 
from F to E. The height of the bridge is spoken of as 
"out" or "in" ; the former when F extends beyond the 
plane, and "in " when F is behind the plane of the lenses. 
(See Figs. 208 and 209.) 

Another good way to take the foregoing measurements 
is to have several ordinary steel frames of different sizes and 



Fig. 210. 

shapes, using whichever one of these seems to fit the best, 
and then making any additional alterations in the measure- 
ments that may be required. 

Third Measurement. — This is the length of the sides or 
temples. This measurement is taken from the top of the 
ear to the plane of the lens, or a horizontal line extending 
out from the eyelashes. 

Fourth Measurement. — The Size of the Lenses. — This 
will depend upon the breadth of the face, the amount of 
space taken up by the bridge, its arms and attachments, as 
also the space occupied by the hinge and attachment of the 
temples. Ordinarily, as stated before, the adult will select 
size O and the child No. 2. 



304 REFRACTION AND HOW TO REFRACT. 

The following blank is a good guide, as covering all 
the necessary measurements as referred to in this descrip- 
tion for ordinary glasses. 

STYLE OF BLANK FOR THE SURGEON TO FOLLOW WHEN 
ORDERING GLASSES FOR HIS PATIENT. 

Patienfs Name, . 

Forward to, 

"\ F 

I f\ i E U \ D 

W D 




Fig. 211. 




R. 



O. D. 

O. S. 



Distance or Near Frames. 
Frames of 



Measurements. 
Spectacles. Eye-glasses. 

Interpupillary distance, ..... Interp'upillary distance, 

Height of bridge, Length of guard, W to T, 

Base of bridge, Width at base, W to D, 

Shape of bridge (see drawing), . . Width at top, T to P, . 

Bridge, "in" or ''out," Length of arm of guards, 

Length of temples, Shape of spring (see drawing 

Size of "eye," Size of "eye," 

Additional notes, 

Date, . 



,M.D. 



STYLE OF FRAMES. 



305 



Style of Frames. — If the glasses are to be worn con- 
stantly, they should be perpendicular or inclined about 5 
degrees from the perpendicular to the front of the eyes. 
(See Fig. 213.) They are spoken of as " distance" frames. 



Fig. 213. 



n 




Fig. 214. 



If the glasses are to be worn only at near-work, then the 
lenses should be tilted downward ; this is known as the 
"near" frame. (See Fig. 214.) 

Fitting Eye-glasses. — The position of the lenses ap- 
plies equally well to eye-glasses. The 
principal measurement is for the nose- 
pieces or guards and the arms or off- 
sets from the guards. (See Fig. 215.) 
The width of the patient's nose where 
W and D, and also T and P, will press, 
depends, of course, upon the length of 
the guard itself — usually about 14 mm. 
It is also necessary to measure the posi- 
tion of the guards relative to the plane 
of the lenses ; that is, whether the arms should be long, 
medium, or short, and whether they are "out" or "in" 
26 




306 refraction and how to refract. 

from the plane of the lenses. The style of spring is usu- 
ally that shown in figure 215. 

Bifocals. — The measurements for bifocals are the same 
as for the spectacle or eye-glass, except the size and shape 
of the segment, and this should never extend above the 
median line of the lens, and seldom to it. 

Quality of Frame. — These are made of silver, steel, 
aluminium, or gold ; the latter are always to be preferred, 
as more durable in every way. Silver and aluminium bend 
easily, and steel frames rust and break. Every surgeon 
who does his own fitting should possess a small screw- 
driver, two pairs of delicate and yet strong pliers (one with 
round and the other with flat ends)-, and also a small rule. 



APPENDIX* 



The Visual Acuity under definite conditions is an 
index of the strength of the necessary spheric lens 
(plus or minus) which will give a vision of ~- or more. 

For several years the writer has been making a careful 
examination of his private cases for the purpose of coming 
to the positive and demonstrable fact as above stated. For in- 
stance, the question which has been decided is this : if an eye, 
hyperopic or myopic, without astigmatism (or an eye with its 
astigmatism corrected with a cylinder), and it has the ability 
to see -§- and its ciliary muscle under the effect of "drops," 
what strength spheric lens would be required to give it normal 
vision ? 

For purposes of study the eyes selected were those which 
obtained standard or more than standard vision with cor- 
recting lenses, and all eyes which from any cause could not 
attain at least —■ vision were excluded from the tests and are 
not a part of the subject-matter under consideration. To 
make these tests reliable two essentials were absolutely neces- 
sary: namely, the ability to read -^- or more and the ciliary 
muscle under the effect of a reliable cycloplegic. 

To begin with, the writer had to work backward, so to 
speak, and in the following manner: the eyes were tested 
at six meters, and with the lenses which gave standard vision, 
the eyes were tested to find out what their visual acuities 

*This paper was read before the Ophthalmic Section of the College of 
Physicians of Philadelphia, Dec. 17, 1908. 

3°7 



3 o8 



REFRACTION AND HOW TO REFRACT. 



would be when plus spheres were placed in front of the cor- 
recting lenses. The spheric lenses used ranged from +0.25 
to +2.50 in quarter diopter intervals. The resulting visions 
obtained are shown in the following table. 



ction with 


correcting 


lenses 






. . . Vision 


VI 


(< <( 




adding +0.25. . 




VI 
VI 

vis 


(< << 


« 


(< 


tt 


+ O.50. 




VI 
VIISS 


<< «< 


tt 


(< 


a 


+ O.75. 


.. « 


VI 
— VIIIS 


M It 


" 


« 


it 


+ I.OO. 


<< 


VI 
— X 


tt «( 


«( 


<« 


tt 


+ I.25. 


«< 


VI 

~" XII 


it a 


«« 


<« 


a 


+ I.50. 


a 


VI 
XV 


<( tt 


<< 


u 


it 


+ I-7S- 


... " 


VI 
XX 


(( tt 
tt tt 


«< 


(C 


it 


+ 2.00. . 
+ 2.25.. 


tt 


VI 
~" XXX 
VI 
LX 



With +2.50 added the vision was less than ^-. 

Studying the table in reverse order it was then found 
that all healthy eyes, hyperopic or myopic (or eyes with the 
astigmatism corrected), and under the effect of "drops" re- 
quired a spheric lens (plus or minus as the case might be) 
consistent with the visual acuity as given in this table. The 
interesting fact was also apparent that a +2.50 spheric 
lens reduced the vision so that the "LX" letter could not be 
distinguished, thus the 2.50 obscured the vision to zero as the 
eye could not detect any letter at six meters. It therefore 
became evident that an eye with the two essential conditions 
mentioned, and having a vision of more than -^, would re- 
quire a spheric lens of less strength than 2.25, and if its 
vision was less than ~ it would require a spheric lens of 
2.50 or stronger. 



APPENDIX. 



309 



To meet the visual conditions shown in Table 1, the 
author had made a series of letters on duplicate cards, and for 
obvious reasons called them "Metric Test Letters." These 
letters conform to the tangent of the angle of five meters 
and of the Gothic character in preference to the English 
block letters. These letters are arranged in series to be 
seen at the varying distances as indicated in meters and the 



- 




~ 


z 




■E 


-L 


p 


F 


K X 


H 


N V 


O L 


F 


Z E 


T V Y 


' D 


C L X 


BNHREZCDO 


- F P A 


R V V 


B T E ~ 


-VYLO 


D C X 


B R F P- 


- R P A L 


C D O 


H N T X '- 


P I- R B 


T A E 


V U H F 


A V X O 


DNHYTRPL. 


" — . ° 


TEPL2 






F« 




— 


E 




z 


F 


L 


p 


H N X K V 


r L 


E O F Z 


D C 


V Y T 


' L X 


-B R N H Z E 


C D O 


- P F 


A R B Y 


V T E - 


- V Y X 


L D C 1 


3 F B R 


-LCD 


O A R P > 


: T N H - 


- TYHRPLDNOXVA - 


FHUVTEA8F 


ILP 


r-=«^ 


T L E P Z D O 





Fig. 216. 



Fig. 217. 



equivalent in feet. The confusion letters are conspicuous 
and the two cards have corresponding letters on the same 
lines but differently arranged. These cards also differ from 
the cards in ordinary use, as there is a line of letters to be 
seen at VI| meters and also at VIIISS meters; the XXV meter 
line as seen on other cards has purposely been omitted. The 
question is often asked why test types are not arranged so as 



3io 



REFRACTION AND HOW TO REFRACT. 



to give a visual acuity at 55, 50, 45, and 40 meters, but the 
uselessness of such letters is easily understood by a review 
of the visual acuities just tabulated. The writer has, however, 
placed one letter on each of the cards here shown (Figs. 216 
and 217), which letters can be seen at 40 meters, but these 
are not a part of the test here described, but will be found con- 
venient in an office with a four meter range. It will be seen 
from the following table that by the arrangement of the size 
of the letters the visual acuity can always be reduced to 
tenths as follows: 



TABLE 


2. 




VI 
ision 15 


equals 


tV 


« VI 
XXX 


a 


A 


u VI 

XX * 


cc 


A 


« VI 
XV 


« 


A 


« VI 
XII 


(i 


A 


« VI 
X 


ti 


A 


u VI 
VIIISS 


" 


A 


« VI 

VIISS 


(« 


A 


« VI 
VI§ 


it 


A 


VI 
VI 


u 


10 

TS 



As the visual acuity is now reducible to tenths it becomes 
a rule of thumb, so to speak, to calculate the strength of the 
correcting spheric lens necessary to produce normal or 
standard vision when it is remembered that +2.50 just re- 
duces the vision of a standard or emmetropic eye to zero, or 
that plus 2.25 gives ^th vision. Take, for instance, a vision 
of ^-th ; that means that the' eye is deficient -^ ths or 
requires y 9 Q-t ns additional to give it x"0" tns or standard sight. 



APPENDIX. 311 

As y-jth of 2.50 is 0.25, then nine times 0.25 would be 2.25, 
the amount of spheric lens required. Or if an eye sees the 
xxx meter letters it has ^-ths vision and requires y 8 o-t ns 
additional to give it f-jfths or normal vision, namely, eight 
times one-tenth of 2.50, which is 2.00 (plus or minus). If 
the vision is y^ths, the eye would require yo^ 13 more* or 
1.50, to give it standard vision. The following table, which 
includes the two just given, shows the strength of the spheric 
lens required in each instance when the visual acuity is ex- 
pressed in tenths. 



TABLE 3. 



English 
Feet. 


Meters. 


Vision at six 
meters. 


Vision in 
tenths. 


197. 


lx 


VI 

LX 


i 
TO 


98.5 


xxx 


VI 
XXX 


2 

To 


65.6 


XX 


VI 
XX 


A 


49.2 


XV 


VI 
XV 


x% 


39-4 


xii 


VI 
XII 


A 


32.8 


X 


VI 
X 


6 

TO 


27.9 


viiiss 


VI 
VIIISS 


tV 


24.6 


viiss 


VI 

vnss 


A 


21.8 


vif 


VI 

vis 


T 9 


19.7 


vi 


VI 
VI 


10 
T~5 


16.4 


V 


VI 

V 


u 


13-9 


hi 


VI 
IV* 


tt 



Spheric lens required for 
VI vision. 
~Vf 
2.25 

2.00 

1-75 
1.50 
I.25 
1. 00 

0-7: 
0.50 
O.25 
0.00 
0.00 



By his previous ophthalmoscopic findings and with his 
retinoscope, and obtaining the patient's near point, etc., 
the observer will know when to employ a plus or a minus 
spheric lens. 



312 



REFRACTION AND HOW TO REFRACT. 



To make the metric test letters of value when the eyes are 
astigmatic, it will be necessary to correct the astigmatism 
with the necessary cylinder before testing the visual acuity. 
For this purpose the writer has also prepared metric lines 
in the form of the clock dial (Fig. 218). This dial has been 
made in two sizes, one for five and the other for a six meter 
distance. These charts vary somewhat from the ordinary 

in that the lines from xii to vi 
and from ix to iii cross each 
other at the center. And also 
the Roman characters conform 
to the tangent of the angle of 
five minutes, as do the lines and 
spaces. While using these charts 
it will be well to ask the patient 
whether the Roman characters 
correspond with the lines in their 
shade of black. As a matter of 
practical experience the writer 
is partial to just such lines and 
spaces as here described, not- 
withstanding the fact that others 
recommend wider lines and 
spaces. In using the metric let- 
ters and lines the writer follows 
the method recommended by Dr. 
J. S. Johnson, of St. Paul, and described in the "Ophthalmic 
Record," Oct., 1901; that is to say, to correct the astigmatism 
with the cylinder as the patient looks at the lines, and when 
they all appear equally black (not necessarily distinct) then 
the visual acuity is taken and the necessary spheric lens 
employed as indicated in the above table. 
To expedite matters and have the patient read the lowest 




Fig. 21* 



APPENDIX. 313 

line of letters which he can see without naming the letters 
from the top of the card, the writer makes use of the red 
and green strips of paper suggested by Dr. Holbrook Lowell, 
of Boston, in his description published in the " Archives of 
Ophthalmology," Vol. xxxv, No. 5, 1906. 

In tabulating the strength of lenses which all patients 
select, it is found to be an interesting fact that about 86 per 
cent, of all patients coming for correcting lenses, select a lens 
not stronger than a four diopter sphere or cylinder or both 
in one or both eyes, so that this 86 per cent, could be suc- 
cessfully refracted with a limited number of lenses in the 
trial case; but the point which is still more interesting is, 
that nearly 70 of this 86 per cent, accept lenses of less than 
2.50 and obtain a vision of -^j- or more in one or both eyes. 
With these facts in mind the value of the metric test letters 
and lines is self-evident. This quick method of arriving 
at the correct lenses may not appeal to the slow ophthalmo- 
logist who criticizes "quick work as poor work," but in 
clinical work at least the assistant has a time-saver which 
he will enjoy. However, when by this method the vision has 
been brought to* normal, the ophthalmologist should not 
prescribe until he has verified the correction by trying the 
various weaker spheres and cylinders as described on page 
229. At this final testing the examiner should use the twelve 
one-hundredths lenses, as no provision has been made in the 
metric letters for a visual acuity in fractions of less than one- 
tenth, as this would have necessitated additional lines of 
letters and a much larger card. 

The metric letters and lines may be used when practising 
the " fogging method," but as most eyes do not entirely relax 
the ciliary muscle when so tested, the visual acuity is liable 
to be one-tenth or sometimes two-tenths more than it would 
be if the eye were under "drops." 
27 



314 REFRACTION AND HOW TO REFRACT. 

In presbyopic cases the metric letters prove very satis- 
factory if there are no structural changes to interfere with 
normal vision. 
To summarize : — 

1. The eye which is being tested should have its ciliary 
muscle at rest with a reliable cycloplegic. 

2. The eye must be capable of obtaining -^- vision or 
more than —-. 

3. All testing must be done at a distance of six meters. 

4. If the eye is astigmatic this must be corrected with the 
necessary cylinder lens before taking the visual acuity and 
adding the spheric lens. 

5. The visual acuity obtained if the eye is hypermetropic 
or myopic (or astigmatism corrected) is an index of the 
strength of the spheric lens required to give -yf vision. 

6. The same lens or lenses which give a vision of -^- will 
at the same time give a vision of more than ~, and without 
any change if the lenses have been carefully selected and the 
eye is capable of seeing more than -^-. 

7. This metric letter testing is of advantage in proving 
the static correction, as shown in Table 1. 

Distance or Range. — For a number of years the writer 
obtained a six meter range in his otherwise small office, by use 
of the plane mirror and reversed letters, as described on page 
75 and Fig. 74, but has abandoned this method for two other 
ranges, one at six meters and the other at twenty meters 
(sixty-six feet.) These distances are obtained by use of the 
side yard. Test cards covered with the best quality of quarter 
inch plate glass and securely framed, so that dampness can- 
not reach them; are fastened to the fence at the distances 
mentioned; and are seen by the patient as he looks through 
the window, also of the best plate glass. The writer takes 
great pleasure in recommending this method of long-range 



APPENDIX. 315 

testing, as it has proven of intestimable value. Tn explana- 
tion of this statement the writer finds that when the correct- 
ing lenses at six meters give a vision of -^j- or more and the 
prescription is written for — 0.25 added to the static refrac- 
tion, occasionally his patients and those of other oculists 
complain of distant objects (beyond twenty feet) looking dim. 
However, when the correcting lenses (static refraction) give a 
vision of -^| or -^ (meters not feet) and — 0.25 is added for 
this unusually long range, none return with the unfavorable 




Fig. 219. 

criticism that "distant objects look blurred" or that they 
"can see better at a distance without glasses." 

Axonometer. — Fig. 219 shows the latest model of this 
useful little instrument used in finding the axis of the band 
of light when using the retinoscope, and is a decided improve- 
ment over the old model as shown in Fig. 155. The broader 
white line is of signal advantage as compared with the 
narrow line. 

Fused Bifocals (Kryptok). — This variety of bifocal is 
also known as "invisible." It is not unlike the bifocal 
shown in Fig. 185. It is made by taking a small circular 
piece of flint glass and by great heat fusing one surface to a 



3 i6 



REFRACTION AND HOW TO REFRACT. 



piece of crown glass; both the crown and flint glass have 
plane surfaces and the crown glass is square in shape. This 
is the form in which the Kryptok Sales 
Company supply the opticians, who 
then grind the various curves to meet the 
conditions of the oculists' prescriptions. 
One Piece Bifocals.— This is not 
unlike the solid or ground bifocals, 
Figs. 187 and 188, but it is now made 
in the toric curve and of much neater 
and more delicate workmanship than 
the previous solid bifocal. This one 
piece bifocal is frequently called "in- 
visible" also, but there is really no bi- 
focal that is absolutely "invisible." 
Close inspection by reflected light will 
generally show where the two edges 
come together. Patients frequently 
think they will avoid seeing where the 
upper and lower corrections come to- 
gether, but they will see it in any bifocal, 
and the term "invisible" applies to the 
friends of the patient, who cannot 
always see that bifocals are being worn. 
Patients of unknown age wearing bi- 
focals of the "invisible" variety enjoy 
the fact that their friends still think 
that they are young (?). 

The Luminous Retinoscope (Fig. 

220; De Zeng Patent). — This latest 

model of luminous retinoscope does away with the tilting 

light as shown in the old model, Figs. 161 and 162, and is 

therefore a much better instrument and much to be preferred. 




Fig. 220. 



NDEX. 



A. 

Abduction, iSo 
Aberration, negative, 175 

positive, 173 
Absorption of light, 12 
Accommodation, 65, 66 

amplitude of, 69, 70 

at different ages, 70 

at rest, 68, 69, 245 

binocular, 84 

cramp of, 266, 267 

diminution of, 70 

in emmetropia, 70, 71 

in hyperopia, 71, 72 

in myopia, 72, 73 

in presbyopia, 70 

mechanism of, 66, 67 

muscle of, 66, 67 

observer's, 96, 97 

paralysis of, 216, 217, 218 

patient's, 96, 158 

range of, 69, 70 

relaxed, 70, 97 
proof of, 245 

spasm of, 266, 267 
Acuteness in astigmatism, 78 

in emmetropia, 103 

in hyperopia, 106 

in myopia, 115 

of vision, 63, 64, 78, 79 

record of, 78, 79 
Adduction, 180, 181 
Aerial image, 100 
Age, 70, 224, 228 
Albino, 93 

Alternating strabismus, 196 
Amblyopia, 157 
Amblvoscope, 203, 204 
Ametrometer, 148, 149 
Ametropia, 105 

axial, 105, 119, 120, 121 

curvature, 105, 122 
Angle alpha, 87 



Angle, critical, 20 

gamma, 85 

limiting, 20, 62, 63 

meter, 83, 84 

of convergence, 8^ 

of deviation, 24 

of five minutes, 64 

of incidence, 22 

of refraction, 21, 22, 23 

of strabismus, 199 

of view, 61 
Anisometropia, 280, 281 

classification of, 280, 281 

correction of, 281, 282 
Anterior focal point, 60 

focus, 60 
Apex of prism, 23 
Appendix, 307 
Aphakia, 157, 278 

causes of, 278 

diagnosis of, 278, 279 

treatment of, 279 
Aqueous humor, 60 
Asthenopia, 219, 220 

accommodative, 221, 222 . 

muscular, 183, 184, 221 

retinal, 220, 221 

treatment of, 220, 221, 222, 223 
Astigmatic charts, 137, 312 

clock-dial, 138, 139 

lens, 123 
Astigmatism. (See Chapter V.) 
Appendix. 

against the rule, 131 

asymmetric, 130 

causes of, 124 

compound hyperopic, 127, 128, 
256, 257, 258 
myopic, 128, 259, 260, 261 

corneal, 123 

diagnosis of, 133 

estimation of, 124. (See Chap- 
ter VI.) 



3^7 



3i3 



INDEX. 



Astigmatism, heterologous, 132 

heteronymous, 132 

homologous, 132 

homonymous, 132 

irregular, 124, 265, 266 

lenticular, 124 

mixed, 128, 129, 261, 262, 263, 
264 

physiologic, 124, 125 

principal meridians of, 126 

regular, 125 

shape of disc in, 153 

simple hyperopic, 126, 250, 251, 
252, 253 
myopic, 127, 254, 255, 256 

statistics of, 243 

symmetric, 129, 130 

symptoms of, 133 

tests for, 133, 309 

treatment of, 250, 251, 252, 253 

with the rule, 131 
Astigmia, 122 
Astigmic, 122 
Atropin, 213 
Axiom, 157 
Axis of astigmatism, 126 

of cylinder, 44 

optic, 60, 85 

principal, 15, 32 

secondary, 15, 36 

visual, 85, 86 
Axonometer, 170, 315 

B. 

Band of light, 169, 315 

Base of prism, 23 

Beam of light, 11 

Biconcave lens, 31, 55 

Biconvex lens, 30, 54 

Bifocals, 283, 284, 285, 286, 287, 

3 X 5> 3 l6 
Binocular accommodation, 84 

fixation, 84 

Blepharitis, no 

Borsch, 286 

Brachymetropia, 113 

Briicke, muscle of, 66 

Burnett, 122 

c. 

Camera, 65 
Capsule, 68 
Cardinal points, 59, 60 



Cards, 74, 75, 76, 77, 309 
Cataract, 276 
Catoptrics, 9 
Center of fixation, 86 

of rotation, 86 
Centering of lenses, 294 
Chalazion, 124 
Choroid, 94, 95 
Chromo-aberration test, 145, 146, 

147, 148 
Ciliary body, 66 
muscle, 66 

anatomy of, 66 
Cobalt -blue glass, 113, 145, 146, 

147, 148 
Cocain, 91, 215 
Compound system, 60 
Concave lenses, 31, 37 
mirror, 15, 16, 17 

in retinoscopy, 159 
Concomitant squint, 196 
Condensing lens, 99 
Confusion letters, 134, 309 
Conic cornea, 172 
Conjugate foci, 34 
Conjunctiva, no 
Convergence, 83, 84 
amplitude of, 85 
angle of, 84 

insufficiency of, 183, 184 
negative, 87 
positive, 86 
range of, 84, 85, 86 
Convergent strabismus, 195 
Convex lenses, 30 
Coquilles, 212 
Corneal reflex test, 133 
Cover chimney, 159 

test, 184 
Cramp of accommodation, 218, 266, 

267 
Cretes' prism. 189 
Crossed diplopia, 1 79 
Crystalline lens, 68 
Cycloplegia, 216, 217, 218 
Cycloplegics, 158, 208, 209, 210, 211, 

212, 213, 215, 216 
Cylinder lenses, 43 

action of, 43, 44 
axis of, 43, 230 
combination of, 49 
crossed, 52, 231 
neutralization of, 56, 57 



INDEX. 



319 



D. 

Dark glasses, 212 

room, 91 
Daturin, 213 

Decentering lenses, 295, 296 
Deorsumduction, 1S1 
DeSchweinitz, 191 
DeZeng, 102, 175, 316 
Deviation, angle of, 24 

estimating size of, 199, 200, 201 

in strabismus, 199 
Diopter, 41, 42 
Dioptrics, 9 
Dioptric system, 60 
Diplopia, 29, 178, 179 

correction of, 29 
Direct method, 90, 91, 153, 154, T55 
Disc, optic, 93 

perforated, 141, 142 

pin-hole, 47, 266 

Placido's, 134, 135 

shape of optic, 93 
Distance, 314 
Distant type, 81, 82, 309 
Divergence, 83, 196, 197 
Divergent strabismus, 196 
Duboisin, 213 
Dynamic refraction, 234, 235 

E. 

Elasticity of lens, 68 
Electric light, 102, 175, 220 
blindness from, 220 
Elongation of eyeball, 242 
Epilepsy, 222 
Emergent rays, 10 
Emmetropia, 103, appendix 

description of, 103, 104, 105 
Erect image, 94, 95, 96 
Esophoria, 184 

diagnosis of, 185 

treatment of, 189, 193, 194 
Esotropia, 18, 195 
Exophoria, 184, 268, 269 

diagnosis of, 185 

treatment of, 189 
Exotropia, 196 
Eye (frontispiece), 59, 60 
Eye drops, 208 

emmetropic, 60 

glasses, 292 

hyperopic, 71 



Eye, myopic, 113 
schematic, 156 
section of (frontispiece), 
standard, 59, 60 
-strain, 219, 220, 221, 222, 223 



F. 

Fa.ce, asymmetric, 130 

broad, 300 

narrow, 300 
Facial illumination, 162 
Far point, 68, 69 
Finger exercise, 191 
Fitting of spectacles. (See Chap- 

. ter XII.) 
Focal interval, 123 

length, 23 

points, 59, 60 
Focus, 12, 15 

anterior, 33 

conjugate, 34 

negative, 12, 35 

ordinary, 35 

positive, 12, 15 

posterior, ^t, 

principal, 15, 33 

real, 12, 33 

virtual, 12, 35 
Fogging method^ 232, 233, 234, 313 
Form of illumination, 165 
Formation of images, 38, 39, 40 
Fox, 283 
Fusion tubes, 203, 204 



Glass, crown, 22 

flint, 22 
Glasses. (See Lenses.) 
Glaucoma, 211 
Gould, 77, 191 
Green, 137 



H. 

Helmholtz, 103 

Heredity, 108, 116 
Heterometropia, 279, 280 
Heteronymous images, 179 
Heterophoria, 182 



320 



INDEX. 



History, 224, 225 

Homatropin, 91, 213, 214, 215, 216 
Homonymous images, 178 
How to refract. (See Chapter IX.) 
Hyoscyamin, 213 
Hyperesophoria, 182 
Hyperexophoria, 182 
Hypermetropia, 36, 106 
Hyperopia, 36, 71, 106, 244, 245, 
246, 247 

absolute, 109 

acquired, 278 

amount of, 72, 121 

axial, 105 

causes of, 108 

description of, 106, 107, 108 

diagnosis of, 112, 113 

estimation of, 247 

facultative, 108 

latent, 109 

length of eyeball in, 106, 247 

manifest, 109 

relative, 109 

symptoms of, no 

total, 109 

treatment of, 237, 245 
Hyperopic astigmatism, 126, 127, 

128 
Hyperphoria, 179, 186, 194 
Hypertropia, 182, 196 



Illiterate card, 75, 76 

Illiterates, 75, 76 

Illuminated area. (See Figs. 86, 

87, 88.) 
Illumination, 162 

facial, 162 

retinal, 162 
Images, 177 

catroptric, 9 

crossed, 179 

formation of, 38, 39, 40 

formed by mirrors, 14, 15, 16 

heteronymous, 179 

homonymous, 178 

in astigmatism, 126 to 129 

in emmetropia, 65, 100, 101 

in hyperopia, 65 

in myopia, 65 

in retinoscopy, 164, 165 



Images, inverted, 17, 99, 100 

on cornea, 162 

on lens, 162 

real, 16, 38 

retinal, 61, 98 

size of, 61, 62, 64 

virtual, 16, 18, 38 
Imbalance, 182 
Inch system, 41, 42, 43 
Index of refraction, 20, 22 
Indirect method, 99, 100, 155 
Infinity, 35, 68 
Infraduction, 181 
Insufficiencies, 183, 184 
Intensity of lids, 9, 10 
Interval, focal, 123 

of Sturm, 123 
Inversion, 14 
Iris in accommodation, 68 

in hyperopia, 112 

in myopia, 118 
Irregular astigmatism of the cornea, 
125 
of the lens, 124 



Jackson, 209 
Johnson, 312 



J. 



K. 



Keratometer, 134, 152 
Keratoscope, 134 
Kindergarten card, 76 
Kryptok, 315, 316 



Length of eyeball, 59 

in emmetropia, 59 
in hyperopia, 106 
in myopia, 106 
in standard eye, 105 
Lens, crystalline, 68 
Lenses, 29, 30, 31, 297, 298, 299 
acromatic, 286 
action of, 31, 32, ^, 35, 36, 37, 

38, 39, 40, 41 
astigmatic, 123 
biconcave, 31, 55 
biconvex, 30, 54 
bifocal, 283, 284.. 285, 286, 287 



INDEX. 



321 



Lenses, collective, 30 

combination, 47, 48, 49, 50, 51, 

5 2 > 53> 54 

crystal, 288 

cylindric, 29, 43, 56, 57 

decentered, 295, 206 

dioptric, 41, 42, 43 

inch, 41, 43 

magnifying, 30 

meniscus, 30 

minifying, 30, 31 

negative, 30, 31 

numeration of, 41, 42, 43 

perimetric, 394 

periscopic, 30 

planoconcave, 31 

planoconvex, 30 

prismatic, 53, 54 

spheric, 29, 30 

spherocylindric, 45 

tinted, 294 

toric, 290 

trifocal, 295 
Ligamentum pectinatum, 66 
Light, 9, 91, 241 

and shade, 79, 163 

intensity of, 9, 10 

-screen, 159 

sense, 79 

velocity of, 9 
Lorgnettes, 289 
Loring, 84, 88 
Lowell, 313 

M. 
Macula, 59, 85 
Maddox rod, 188 
Malingerer, 28 

Manifest refraction, 234, 235 
Meniscus, 30 
Meridians, 132 
Meter, 41, 42 

angle, 83, 84 
Metric system, 41, 42, 43 

test letters, 309 
lines, 312 
Mires, 152, 153 
Mirror, 14 

concave, 15, 16, 17 

convex, 15, 17, 18 

plane, 14, 158 

reflection from, 14, 15, 16, 17 
Mixed astigmatism, 128, 129 
Mo Yemen s of mirror, 161, 162 



Mulatto, 162 

Muscles. (See Chapter VII.) 

ciliary, 66, 67 

Muller's, 67 

picture of (frontispiece). 
Mydriatics, 208 
Myopia, 113 

axial, 113 

causes of, 115, 116, 117 

description of, 113, 114, 115, 
248 

diagnosis of, 118, 119 

estimation of, 249 

image in, 65 

length of eyeball in, 121 

ophthalmoscopic appearances 
in, 248 

progressive, 242 

symptoms of, 117, 118 

treatment of, 238, 239, 240, 249 



N. 

Near point, 80, 228 

determination of, 80 

Nebula, 265 

Negative aberration, 173 
angle, 87 

Nerve, optic, 93 

color of, 93 

shape of, in astigmatism, 

153 
size of, in hyperopia, 96 
in myopia, 97 
Nettleship, 121 

Neutralizing lenses, 54, 55, 56, 57, 58 
Nodal points, 36, ^j 
Nystagmus, 157 



Observer, 91, 92 
Occupation, 224 
Ocular gymnastics, 203 
Opacities, 125, 265 
Ophthalmometer, 125, 149, 150, 151 
Ophthalmoplegia, 216 
Ophthalmoscope, 88, 113, 119 

how to use, 90 

luminous, 101, 102 
Optic axis, 60, 85 

center, 37, 54, 55 

disc, 93 



322 



INDEX. 



Optics, 9 
Orbit, 116 
Orthophoria, 177 



Paralysis of accommodation, 216 

causes of, 217 

treatment of, 218 
Pencil, converging, 11, 12 

diverging, 12 
Perforated disc, 141 
Perimeter, 201 
Periodic squint, 196 
Phenomena of light, 12 
Phorometer, 190 
Phorometry, 180 
Pinc-nez, 292 
Pin-hole disc, 47 
Placido's disc, 134, 135 
Pointed line test, 141 
Points, cardinal, 59, 60 

nodal, 37 

of reversal, 163 

principal, 60 
Postcycloplegic, 235, 236 
Pray's letters, 142 
Presbyopia, 271, 272 

age of, 272 

causes of, 272 

description of, 272, 273 

diagnosis of, 273 

glasses for, 274, 275, 276, 277 

symptoms of, 273 
Prescription writing, 53, 54 
Principal axis, 15, 32 

focus, 15, 32 

points, 59, 60 
Prism-diopters, 25, 26 

exercises, 191, 192 

rotary, 189 

Wollaston, 149 
Prisms, 23 

action of, 23, 24 

centrads, 25 

neutralization of, 26 

numeration of, 25 

uses of, 28, 29 
Punctum proximum, 69 

determination of, 79, 80, 81 
in emmetropia, 69, 70 
in hyperopia, 72 



Punctum proximum in myopia, 73 
remotum, 68, 69 

determination of, 72 
in emmetropia, 69 
in hyperopia, 71, 72 
in myopia, 72, 73 
negative, 72 
positive, 73 
Pupil, size of, in emmetropia, 1 1 
in hyperopia, 244 
in myopia, 248 



R. 

Randall, 74 
Range, 314, 315 

of accommodation, 69, 70 
in emmetropia, 70, 71 
in hyperopia, 71, 72 
in myopia, 72, 73 
of convergence, 85 
Rays, 10, 32 

convergent, n 
divergent, 10, 95 
emergent, 10 
incident, 10 
parallel, 10 
reflected, 10 
refracted, 10 
Reflection, 13 

by mirrors, 13 
laws of, 13 
Refraction, 18, appendix 

applied. (See Chapter X.) 

by cylinders, 43, 44 

by prisms, 23, 24 

by spheres, 31 to 41 inclusive 

how to refract. (See Chapter 

IX.) 
index of, 22 
laws of, 19 
Regular astigmatism. (See Astig- 
matism.) 
Reisner, 173, 174 
Retina, 69, 94 

Retinal asthenopia. (See Astheno- 
pia.) 
illumination, 162 
image in astigmatism, 123 
in emmetropia, 65 
in hyperopia, 65 
in myopia, 65 
reflex, 92 



INDEX. 



;>2 



Retinoscope, 159, 175, 316 
Retinoscopy. (See Chapter VI.) 
Risley, 117, 189 
Rods and cones, 74 
Rod test, 188 
Room, 91 



S. 

Scheixer's test, 113, 119, 143, 144 
Schematic eye, 156 
Scissor movement, 172 
Scopolamin, 213 
Second sight, 276 
Shadow test. (See Chapter VI.) 
Shadows in retinoscopy, 163 
Simple hyperopic astigmatism, 126 

myopic astigmatism, 127 
Snellen, 75 
Snow blindness, 220 
Spasm of accommodation, 218 
causes, 218, 219 
symptoms, 219 
treatment of, 219 

clonic, 218 

tonic, 218 
Spectacles. (See Chapter XII.) 

for adults, 298 

for aphakia, 278, 279 

for astigmatism, 292 

for children, 298 

for hyperopia, 295 

for myopia, 300 

for presbyopia, 295, 298, 299, 
300 

for strabismus, 290 

measurements for, 300, 301 
302, 303, 304, 305, 306 
Spheres. (See Lenses.) 
Squint. (See Strabismus.) 
Standard eye, 59, 307, 308 
Static refraction, 236, 237 
Stenopeic slit, 135, 136 
Stevens, 182, 189 
Strabismometer, 200 
Strabismus, 195 

alternating, 196 

amount of, 28, 199, 200, 201 

angle of, too 

apparent angle of, 199 

causes of, 196, 197, 198, 199 

concomitant, i</> 



Strabismus, constant, 196 

convergent, 195 

divergent, 196 

monolateral, 196 

paralytic, 196 

periodic, 196 

treatment of, 28, 201, 202, 203, 
204, 205, 206, 207 
Sturm, interval of, 123 
Supraduction, 181 
Surfaces of cylinders, 43 

of mirrors, 14, 15, 16, 17, 18 

of prisms, 23 

of spheres, 54, 55 

sursumduction, 181 
Symptoms of aphakia, 278, 279 

of asthenopia, 219, 220 

of astigmatism, 124 

of hyperopia, no 

of myopia, 117, 118 

of presbyopia, 273 



T. 

Table of amplitude of accommo- 
dation, 70 

of axial length of eyeball, 121 

of indexes, 22 

of lenses, 43 

of near points, 272 

of prisms, 27 
Targets, 152, 153 
Tenotomy, 124, 194, 195, 206, 207 
Test for aphakia, 278, 279 

for astigmatism, 133 

for hyperopia, 112, 113 

for malingering, 28 

for muscles. (See Chapter 

VII.) 

for myopia, 118, irg 

for near point, 81, 82 

for vision, 78, 79, appendix 

-letters, 74, 75. 76, 77. 309 

-type, 81, 82 
Thomson's ametrom; iter, 113, 119 

148, 149 
Tinted glasses, 294 
Toric, 290 
Trial-case, 45, 46 
Trial-frame, 47, 48 
Trifocals, 295 



324 



INDEX. 



V. 

Vacuum, 9, 21 
Virtual focus, 12, 35 

_ images, 12, 35 
Vision, acuteness of, 62, 63, 78, 79 
binocular, 177 

determination of, 73, 74, 77, 78, 
appendix 
Visual acuity, 62, 63, 77, 78, 79, 307, 
308, 309, 310, 311, 312, 314 



Visual angle, 61, 62 
axis, 60 

normal, 63, 64 



W. 



Wallace, 75 
Wollaston, 149 
Worth, 203, 204 



fep 24 m° 



